The reason Prof. X and Magneto (they are about the same age, maybe Magneto is a bit younger) are physically younger than they should be is they got rejuvenated at least one time:
1) Magneto turned a toddler during the Mutant X storyline of the Defenders in the 70s (when there was no X-men comic)
2) Prof X being healed a few times by Shi'ar, body rebuilt after death, etc.
IMHO, when mutants (and some super-powered hero) use their powers they tend to return to their original mind/brain state (and I suppose adult body configuration). It was stated in a X-men story where Magneto had brain-washed the X-men and they, anyway, broke out of mind control. Same reason Magneto tend to become paranoid psychotic when he use his power too much for too long.
It is unfortunate we can not have the accurate aged characters just because [reasons] when others have no problem with the same age characters in writing (Game of Thrones or the Bible do not matter) just because [reasons].
For reference: age of consent in England was 10 after 1000AD, 12 after 1100 AD up until the end of 1800s and then they rose it up to 16 in a few decades); and, of course, when police in involved with some specific gangs they consider 12 "informed enough and able to choose" their lifestyle.
Evil Hypocrites everywhere.
