User blog comment:FairyRaven/Fairy Tail Chapter 325: Unity, Review and Commentary/@comment-49.176.34.54-20130330054337/@comment-7418318-20130331073521

Come to think of it... their bodies changed into something hard enough to block bladed weapons with their bare hands (Gajeel is pretty obvious in this regard, but even Natsu blocked Erigos scythe with his bare forearm).

Could it be that they didn't actually trigger the age rule of the barrier, but the other one? "People over 80 [b]and statues[/b] may not pass". Could it be possible that the spell detected the changes to the Dragon Slayers and "misinterpreted" (for lack of a better term) those changes as being part of the petrification process?