User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-4164634-20141116163251

I think Natsu is 18-19 years old and Gajeel is between 19-22 years old. The reason they couldn't go through Freed's ruins is because they use a form of Lost Magic. So probably if you put a God slayer in there to try to get through they would have similar issues.