User blog comment:IamJakuhoRaikoben/Chapter 441: Caracole Island, Review./@comment-24536822-20150622051441/@comment-7418318-20150622171837

It's not about age.

Gajeel got his motion sickness way past 13 years in age. It's more about a certain power or skill level it would seem. That's also what Sting told Gajeel in the GMG.