It's simple and fits perfectly with the AGP profile: fetishization. MtFs romanticize and fetishize young and teen girlhood that they were excluded from because they were boys. To them at that age girls are a bit like mystical creatures and through media, their own assumptions, and possibly incel influences, they conclude that being a young girl is life on easy mode. They feel jilted they missed out on that. MtFs pursue femininity. This is why a lot of them do not dress and act their age. They also daydream about stereotypical girl rite of passages like sleepovers, first menstruation, girls day out, makeovers, dress up, tea time, etc, and insert themselves into those daydreams. Older MtFs want to live vicariously through younger MtFs like how a narc mother wants her daughter to have the childhood she always wanted. They also probably pity the young boys and think that they will be in a better place if they trooned out.
That's why a newly trooned out dude will ask his female friends for help with clothes/makeup instead of using the damn internet. He wants to experience what's he's fetishized for so long. And even when he gets advice he probably won't follow it as it will conflict with his mental image of how he thinks women dress: anime girls, goth girls, or egirls. That's why a troon almost always dresses like one of those three.
Sidenote, I have rarely seen a troon dress like a stereotypical, Starbucks loving, white shirt with dark jeans and tan knee high boots white girl. Somehow it's just too cis for them.
Anyways, FtMs just don't fetishize young and teen boyhood. There isn't a much of a concrete image of the stereotypical boyhood compared to the stereotypical girlhood. Boys come in many shapes and sizes while girls are constrained to one unreachable ideal (cough that's gender and it's used to oppress women). The young FtMs are escaping femininity, not pursuing masculinity. I'm not too well versed on the mentality of older FtMs, but if I were to guess it's because as they grew older they didn't fit into society's ideal woman: young, lean, beautiful. So they concluded that the must no longer be a woman and must be a man. I think that these older women look at younger women and view them in a better place compared to themselves. The young women fit society's view of a woman more than them and are likely to be treated well, so there's no need to groom them to troon out.