Older transitioners took so long, for the same reason why we definitely need to keep the "Before and After" thread.
Before the internet existed in its current state, the only information you ever got about trans people was from the mainstream media's overly-biased perspective, which basically painted all trans women as nothing more than being men who got surgery on their bodies to superficially resemble women. I believed this lie until I stumbled upon this very site's "Before and After" topic, and transition videos on Youtube.
Even as a kid in the late 90s and early-2000s, when information was readily available on SRS and the existence of trans people, they were still stigmatized as being freaks, having their transitions made to appear superficial and fake, and they never got a chance to speak for themselves and show the transition process for what it really is. The resources that we have now are truly amazing. Had they been available to all of us when we were teenagers, I guarantee you, a lot of us would have transitioned a LOT younger.