He's seriously 19 dating a 13 year old? Not only is that illegal, but a 13 year old doesn't even have boobs yet... What's the appeal in that?
Honestly, the difference between a 14 year old and a 13 year old is huge... Shit, the difference between a girl at the end of grade 8 and the same girl at the beginning of grade 9 is huge... I remember going into grade 9 and going "Wait what the fuck happened to all these girls?" So really as a 16 year old youngest I'd go is 14, and that's only if it was really worth it...