The easy and obvious answer is the same one everyone else gave - that it is individual to when the person in question is ready, but rather than just echo that sentiment let me take a different approach.
In economics, we are taught, there's a principle in which you analyze a given action under the assumption that there are no other factors which skew the conclusion - which is refereed to as "all other things being equal"
the only way I can say anything relevant about Kate's question is to apply that principle.
AOTBE, the best time to transition falls between 18 and 25,
While one would be very blessed to transition, say, in middle school - there is a real and significant possibility that other things are at play in your gender identity questioning. I wouldn't say i'd discourage such an early transition (particularly for a child who was aware from long before puberty of their identity) - just saying that that factor makes it a less than ideal moment.
After your mid-20's, the complications of life REALLY begin to pile up - relationships, career, et al become much more of a factor.
Plus, the demographic who's (in general) the most likely to be accepting and affirming is the younger one (currently) - but admittedly in the future this factor lessens in importance as the older, less tolerant, generations fade from the scene.
But yeah, all other things are NOT equal and no one should feel like there's a "zone" they HAVE to fall into or they will be "late"
A lot of us ARE late (I'm certain, in hindsight, that if I'd transitioned in my early 20's I'd have been successful and I was "ready" - albeit I'd have had to relocate) but just as many (more?) can look back and say they were NOT. also, circumstances change both ways - i wouldn't have been as ready, or rather my life wouldn't have been ready - for me to transition 15 years ago as I was 25 years ago.
so it's a mistake for anyone who is, for instance, 23 to think "oh crap I have to hurry up before the window closes!!" - you have to be sensitive to the ramifications in your particular set of circumstances.