Well traditionally 11 old enough in Japan. But I figure Rin begged him from 11 to 18 before he gave in, Him waiting having nothing to do with her age just his reluctance to start a relationship like this and in the earlier years she had to convince him he actually desired her.
18 was purely for us modern people although with the massive increase of killing self rates and 50% miserable what we are doing is a near total failure by that standard. in 1700's and stronger one 1800's desire to extend childhood and create retirement was done especially as food revolution made more food and medical improvements lowered the child morality rate from 75% which is why young teens or earlier had to have children there is a reason humans can have children that young.
For the old we have found out that life satisfaction is actually from responsibility and that probably the same with young people keeping them from responsablities bad for them. I'm anti marriage but when they never mention killing self rate or happiness with child brides you know they are happier and kill selves less. Muslim religion requires forplay and bringing the woman to orgasim so it probably the sex and responsality for running a house and children that makes that superior. Still Yuck just less yuck then we have for the average.