User blog comment:IamJakuhoRaikoben/Chapter 450: Alone in all the World, Review./@comment-99.65.101.58-20150901063904/@comment-2140588-20150901084213

This was when Makarov was born, and he's like 88-89 now. Mavis's age was said to be around 118. 118-88 = 30.

So, she was around 30.