have been around on mastodon for years (was one of the first few thousands accounts). it really depends where you look. i'm under 30 and the majority of people i see + interact with are under 30 too.
the "professionals" side of mastodon absolutely does skew more to that age range, but the more shitposty social groups skew way younger, and with a lot of them existing on other instances.
lemmy also has a mix and again depends where you look.
tl;dr i'm not sure i'd totally agree with that 90% statistic