Dude grow up farm boys matured faster than you city boys did.
Five. You were barely potty trained. That is not normal, I don't care if you were from the country. It is not normal. You were molested, and so what if your body thought it was grand.
Did your parents know?
And why weren't his parents watching him more? He was 5!
I agree that he was molested. It is possible that he has blocked out the molestation and substituted the 16 year old girl.
I'm thinking that the 16 y/o was molested, and put the 8 and 5 y/o together as a result.