User blog comment:IamJakuhoRaikoben/Chapter 327: My Part Too, Review./@comment-5113042-20130413152152/@comment-4829046-20130413163655

It was never "stated" officially. And the whole time-travel theory does come into play with age since Jutsu shiki is kinda ancient. BUT you have to keep in mind that when Mashima wrote RAVE  he made one of the characters be able to sleep for decades without aging and still lead an average teenage life(when not fighting of course)