Kitty and the New Mutants are about the same age, early 20's. Legion fits in here as well.
The O5 are late 20's-early 30's (Beast is the oldest, Bobby is the youngest). Emma Frost was supposedly 27 during the Generation X days (as stated by the writer) but that seems to have gone ignored.
Hope is 18 and I'd wager Pixie and her classmates are as well. No idea about Generation Hope, Idie is like 15 and Transonic is 18 at the least. Kenji seems to be in his early 20's and I have no clue about Teon.