This article presents a new account of the construction of the modern male homosexual within late-nineteenth-and early-twentieth-century sexual scientific networks in Britain and Germany. It argues that the construction of the male homosexual as an inborn type enabled sexual scientists to detach homosexuality from long-standing associations of same-sex acts with the corruption of youth. In characterising the inborn homosexual as someone who sought consensual relationships with mature men and whose desires were fixed, unchanging and determined by the gender of the object of attraction, not his age, sexual scientists discounted the view that homosexuality could be caused by childhood/adolescent experiences and rejected the idea that homosexuals were perpetrators of abusive relationships with younger males. Yet, as the article also shows, attempts to assert an age-irrelevant gender-based framework for theorising sexuality did not succeed in sidelining age-differentiated forms of desire. On the contrary, age continued to be theorised within sexual scientific debates in dialogue with intersecting homophile communities who remained invested in the erotics of age. As a whole, the article demonstrates the importance of foregrounding age as a central category of analysis in the history of sexuality. © 2019 John Wiley & Sons Ltd.