User blog comment:RennaGraceCarter/Chapter 327 My Part Too - Review/@comment-7418318-20130414064845/@comment-7418318-20130414080929

I don't think they are actually that old. For that I have three theories. The third one is a bit of a stretch, but the first two seem quite plausible to me.

1) We know that (or at least it was strongly hinted at) the only ways to learn DS magic without implanting Lacryma is being taught directly by a Dragon. Maybe they infuse their pupil with part of their own magic and the runes reacted to that infused part.

2) If Igneel incorporated something of himself (like scales for example) into Natsu's scarf and/or infused it with his magic, it could be that the runes actually reacted to the scarf, not Natsu himself. With Gajeel's personality it's quite possible he carries a similar memento from Metallicana, but doesn't want to show it (just like he pretends to not care about his Dragon).

3) The rule of the runes was "No male over 80 or statue may pass". Dragon Slayers definately had their physique changed by their magic. If that change is working through a similar mechanism as Evergreen's petrification, it' possible the runes triggered because of a misinterpretation of the statue part, rather than the age part of the rule.