User blog comment:IamJakuhoRaikoben/Chapter 373: Life or Death?, Review./@comment-11409322-20140228194902/@comment-24066070-20140301202722

Like Gildarts was alive 400 years ago. Even if he was, Makarov (who has know Gildarts for years) would've notice him not ageing at all.

Tho it may make sense: Igneel couldn't kill him and neither could Acnologia, so then he survived 2 confrontations with a Dragon.

But honestly, that theory is nonsense.