The "3 rule" is for females - I'd be a bit more flexible with males. I've heard of males as young as 10 months fathering a successful clutch. I'd be wary of putting a male in with a female very much larger than him in case things get rough, but that's my only concern.
As Floof says, fertility can be a bit of a lottery with the very young males, so there's probably more likelihood of infertile eggs with a young 'un.