According to the "timeline" of the Marvel Universe, about 13 years has passed since the Fantastic 4 went through their cosmic storm induced mutations. Assuming that maybe a year or two later Peter Parker, then 15, was bitten by the spider, he is about 25 to 26. Scott Summers was already leading the X-Men by the time they met Spidey, and had been for about two years putting him at 19 or 20 at the time, which puts Scott almost at 30 (which sort of screws up the established age of his brother Alex at 6 years younger, because Alex was at his college graduation when we first meet him, so there is no way he is only 24. I'd buy 26, but then that makes Scott in his early 30s and Marvel seems to freak out about it.)
Logically, without Marvel's wonkiness, Scott, Warren, and Emma should be early to mid 30s (34-35), Ororo should be a year or two younger (31-32), Hank a year or older (36-38). Alex, Bobby, Rogue, Piotr and Kurt should be even younger (29-30).