I think nowadays the line really is moving forwards. Two decades ago, me transitining in my early 20ies was considered to be young, since any treatments before age 18 or even age 21 when it comes to GRS was really really hard to get. Nowadays, I would draw the line at around age 14 or 15 maybe. So basically at the time when the voice breaks and beard grows, if you manage to transition before tha, you are early and will only need hormones and genital surgery - after that, the amount of work that has to be done is increasing steadily, so its kind of a mixed phase. At about age 25 or older, it matters less if it is one or two years more and the difference between 40 and 50 is not that much either.