User blog comment:FairyRaven/Fairy Tail Chapter 437: Magnolia, Review and Commentary/@comment-24277053-20150525114500/@comment-9595671-20150525143247

I do recall when Makarov was considered the 4th master after the Phantom battle he considered Erza, but he felt she might have been too young.

And Erza's two years older than Lucy no less.