I think a 16 years old girl is quite mature to make some decisions. My mom married at 16 with a man that was 32, and she was 17 when she had me, so I don't see anything intrinsically wrong here. It would be different if we talk about a 13 years old like on the Jerry Lee Lewis example I posted. But we all know girls mature a lot faster than boys. So what I'm saying is, I would not recommend to do it, but don't automatically think of the kid as "innocent and naive" and the adult as a monster. Yes, it would have been better to get somebody her own age, but hey, it's their choice.