User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-26389992-20150518133328/@comment-9595671-20150518142951

Actually I think the reasoning behind them being unable to pass the runes was because Igneel and Metallicana were inside of them and their age was enough for the runes to block them.

Plus going by the age of Natsu's body before Zeref gave it a new soul in this chapter, it was still very young, if its not alive it can't officially age so while he's 400+ chronologically, he's not that old biologically.