User blog comment:IamJakuhoRaikoben/Chapter 449: Mavis & Zeref, Review./@comment-756329-20150824165310/@comment-24264679-20150825014957

Zeref was still pretty young when he became immortal I would say around 14 or 15. He just acts way more older then Mavis because of his life experiences. Mavis believes in the child hood dream of fairies while Zeref is more realistic and believes the world is corrupt and that life sometimes can be a you know what.