As one of these 20-somethings that you speak of... (was 27 when I went to my first support group,)
I know the first time I went to a support group, and saw that it was almost completely dominated by older-transitioning MtFs, I really felt isolated. Like, they were all talking about older-transitioner concerns... coming out on the job, kids, family, and they all seemed WAY more stereotypically feminine than me, with almost every single one of them wearing dresses, heels, and makeup, acting in overtly-stereotypical-feminine manners, and constantly talking about all of the things that proved that they were really women. So I'll be honest, I really didn't feel like I belonged there. I was kinda expecting it to feel like the groups of female friends I've hung out with my entire life, basically just people out living their lives, acting like what I as a young person see as a "typical woman," people who just so happen to be trans and female getting together to hang out and talk about whatever. I didn't expect it to feel like some secretive collection of cross-dressers decked out in stereotypically-feminine attire talking and obsessing about transition as if it was an AA meeting. (I know that's not what they actually were, but it's what it felt like to me at the time.)
Basically, because they clashed so much with what my expectations of what makes someone female was, I felt like I didn't belong there. I was used to hanging around low-key tomboys my whole life, girls whose gender was more incidental, who wore jeans and sweatshirts, not people who looked like they were trying to be on the cover of Better Homes & Gardens. It was a real generational clash for me.