User blog comment:Real Godisme/Chapter 479/@comment-73.191.17.103-20160328163101/@comment-7418318-20160328163626

No, biologically Mavis is 13. That's the moment when she contracted the curse and thus stopped aging. If I remember correctly, Zeref got his curse at 14, but I'm not sure.

The likely reason why she says she's in her twenties is, because that's when she died (while having the body of a 13 year old).

Too many dates with her... a 13 year old body that died at 20-something and still somewhat exists as a ghostlike being 108 years later.