User blog comment:FairyRaven/Fairy Tail Chapter 438: The Seventh Guild Master, Review and Commentary/@comment-4933386-20150601123144/@comment-7418318-20150601194545

I think Mavis body still ages in terms of cell-regeneration and all the stuff that determins life-span, but her appearance got locked to that of a child. I doubt that Last Ages would leave her unharmed.

I think she really is only in Mavis' mind. To me that look she gave Mavis when Yuriy told her the truth looked more like an appologetic and melancholic one. More like "sorry, but it's true, I'm not real" rather than an "That's not entirely the case, I'm real, but not Human."... but that's just my interpretation... I could be wrong here.