User blog comment:IamJakuhoRaikoben/Chapter 450: Alone in all the World, Review./@comment-35340256-20150831151802/@comment-9595671-20150831160117

We know he was a kid when Natsu died atleast, and as Natsu looked to b a newborn I'd say he had to be atleast 3-4 years older. So being 15-17 when the curse was put on him seems to work.