The present study showed that over almost two decades, the distribution of the age of clinic referred transgender adolescents was not evenly distributed, but was distributed in a younger group presenting around the age of 11/12 years (median age 11.95 years) and an older group presenting around the age of 16/17 years (median age 16.25 years). Relatively more adolescents belonged to the older presenting group. This observation is comparable to other studies [19, 20], but this was the first study that determined the cut off between both groups at age 13.9 based on observation of the data, whereas other studies split the groups based on assumed puberty staging or by dividing the group based on start with puberty blockers (mean age 11.2) or GAH (mean age 16) [19, 20]. Like the other studies, the present study also revealed several differences between the younger and the older presenting group. First, both groups had more birth-assigned females, but in the younger group, the sex ratio was 1:1.43 favoring birth-assigned females, whereas in older presenters this was 1:2.35. Our findings of a different gender ratio in younger presenting youth compared to older presenting youth might suggest that different developmental pathways may exist for birth-assigned males compared to birth-assigned females, which deserves further study. In addition, a larger part of the younger presenters lived with both biological parents, whereas the older presenters more often came from divorced families or other living circumstances. Of notice, younger adolescents more frequently were diagnosed with gender dysphoria and started with GAMT. Further, younger presenters showed higher levels of gender nonconformity in childhood. Finally, older presenters were more dissatisfied with most aspects of their bodies.

The present study showed that younger and older presenting youth differed in various demographic aspects. One was that there were relatively more birth-assigned females in the older presenting group. This is of interest and might explain that the ‘shift’ in sex ratio and the overrepresentation of birth-assigned females that was observed in several other studies, concerns, for a fairly large part, the older presenters [7, 33]. One hypothesis for this overrepresentation is that it is more accepted for birth-assigned females to present themselves in their preferred gender compared with birth-assigned males [34, 35].

It is likely that not only the age at which adolescents discover their gender incongruence affects the time point at which adolescents are referred to a gender service. The age at which young persons are ready to be open about their identity probably plays a role, as well as how the social environment reacts [36]. It is notable that there are significantly more adolescents in the older presenting group who do not live with both biological parents. It is possible that it was more difficult and took more time for the parents of these adolescents to get the same perspective on how to best help their child which resulted in a more delayed reference to a gender service. So, older presenting youth may have lacked the support and help of parents that younger adolescents depend upon to be able to come out and be referred to a specialized gender service. However, it could also be that the adolescents in the older presenting group were less likely to live with both biological parents because more time has passed for these adolescents in which their family situation could have changed compared with the younger presenting group. Finally, one hypothesis could be that divorce of parents contributes to more mental health difficulties which may have interfered with a referral to a gender service.

The study of Sorbara et al. showed that older age and late pubertal stage are associated with worse mental health among gender incongruent youth presenting to a transgender service [19]. And although the study by Sorbara et al. only included adolescents who had a gender dysphoria diagnosis, it could be that the result that older age is associated with more mental health difficulties also applies for adolescents who may not (yet) have a gender dysphoria diagnosis but are seeking GAMT. It may be that co-existing mental health difficulties in older referrals are related to a less unequivocal gender identity exploration in which relatively more adolescents follow a path of which GAMT is not a part. This could be a reason that in the older presenting group fewer individuals received a gender dysphoria diagnosis and fewer went on with GAMT. Apparently, of the older referrals, fewer adolescents fulfilled the criteria of a gender dysphoria diagnosis, no indication for GAMT could be made during the exploratory psychological trajectory, or the adolescents refrained from a medical gender-affirming trajectory. It is important that future research focuses on this.

Our finding that there are two peaks in the age distribution of referrals and the differences in demographic characteristics between the younger and older presenting group could indicate that there are different developmental trajectories leading to gender incongruence in adolescence and referral for early (in contrast to adulthood) GAMT. Although the RCGI scores of adolescents from both the younger presenting group and the older presenting group were, on average, relatively low, indicating a high level of gender nonconformity during childhood, the mean RCGI score of adolescents from the younger presenting group was significantly lower in the current study. So during pre-pubertal childhood, the younger presenters showed stronger preference for (stereotypically) gender nonconforming toys and playmates and had a more gender nonconforming appearance compared with older presenters. It might well be that this more extreme gender nonconformity led parents and their children to seek GAMT at younger ages. In contrast, in the older presenting group, childhood gender nonconformity was on average less extreme, so there was probably less reason to seek early help from gender specialists. This group possibly needed more time to realize that their gender identity did not align with their birth-assigned gender and GAMT was desired. The physical changes due to puberty probably might have been essential in this, as our results demonstrate that adolescents in the older presenting group showed more body dissatisfaction compared with the adolescents in the younger presenting group. Furthermore, peer experiences might also play a role as we know from research on developmental pathways in pre-pubertal gender nonconformity [37]; because adolescents encounter more diverse people around puberty, when they start high school, adolescents from the older presenting group may have been in a better position to explore the full range of gender diversity and to figure out which identity fitted them best. Finally, the adolescents in the older presenting group may have lacked the family support that some younger presenting adolescents get [36]. The fact that a larger percentage of these adolescents came from divorced families or other living circumstances may have made it more challenging to seek appropriate care.

Although the results of the present study suggest that there may be different developmental in adolescents that lead to seeking gender-affirming medical care, our data do not allow us to conclude whether or not this suggested ‘ROGD’ subtype exists. Our results show that there was gender nonconformity in childhood in older presenters, although less extreme than in the younger presenting group, which speaks against this suggested subtype. However, we did not evaluate other hypothesized factors that would be associated with ‘ROGD’, such as mental health difficulties. Furthermore, we did not examine how gradual or sudden the onset was. A Canadian study recently examined whether they could identify the phenomenon of 'ROGD' in their clinical population (N = 173). They concluded that there was no 'ROGD' because the vast majority (68–86%) did not have 'recent gender knowledge' (realized their gender was different from what other people called them) and because those who did have 'recent gender knowledge' showed relatively less anxiety severity/impairment [38]. In response to this study, Littman pointed out that Bauer et al. had not used the correct definition of ‘ROGD’ because, ‘ROGD’ would not be related to having a short history of gender incongruence, but to not having gender incongruence before puberty [39]. More studies using both self and parent report measures would be needed to gain better insight in the existence of the ‘ROGD’ subtype.

This study has several clinical implications. The differences in demographic, diagnostic, and treatment characteristics, childhood gender nonconformity, and body image among adolescents from the older and younger presenting group argues for more tailored care. To ensure that each adolescent receives the treatment that best suits them, it is important to thoroughly explore all aspects of gender and general functioning with all adolescents before making decisions about further treatment [40]. The conclusion of a previous study that gender-affirming treatment earlier in life may have benefits is not necessarily founded for everyone [20]. Despite the availability of puberty blockers in the Netherlands since 2000, the largest proportion of adolescents are older before being referred to a gender service, and the majority still comes in adulthood [4]. This may be due to social or environmental factors but could also be due to intrapersonal factors.

Our results should be viewed in light of some limitations. To begin with, in this study we did not examine exactly when and how gender incongruence emerged in adolescents. The measurement instrument for gender nonconformity in childhood is also retrospective, which could possibly result in recall bias. In addition, we did not measure whether adolescents received social support although this may be important for whether or not they were referred to a gender service at an early age. Besides, this study did not evaluate mental health difficulties and whether they differed between younger and older presenters. Furthermore, it was not tracked whether participants identified outside the binary spectrum. Another limitation of this study is that it is a cross-sectional design. The younger presenting group includes different individuals than the older presenting group and, therefore, it is unknown what the effect of age is within a person and no conclusion can be drawn with regard to causal or time-related pathways. Finally, the adolescents in this study are part of a clinical sample. Therefore, we do not know if these findings can be generalized to transgender adolescents who do not enroll in a clinic or present to different gender identity specialty services around the world.