Why are 20s considered the "ideal years" to transition, and everything else after that is "later"?

That’s how it comes off to me, and it just seems kinda unfair. 30 isn't "old" is it? It doesn't seem like it is, and not everyone has the resources to be able to transition in their 20s. Transitioning in your 30s isn't considered that "late" is it...?

I'm not disclosing my actual age, but I'm still in my 20s myself, and I don't want to be considered an "older" trans person once I hit 30. I already hate being considered a "late bloomer". Maybe that's why it irks me.