User blog comment:Miskos3/Fairy Tail Chapters 532 & 533, Review/@comment-4449950-20170424084451/@comment-4449950-20170424105229

As far as I recall his age at the time of becoming immortal has never been stated, just that it happened around 400 years ago from the current point in time. Given his appearance as Mashima draws him (his height, build, etc.) I've always assumed him to be 18 at the youngest.

Unless it was stated, in which case my bad, but I still don't like the pair and we should probably update Zeref's page with that info.