The good thing about having sex young, especially when its both partners first time, there will most likely be no chance of contracting a sexually transmitted disease, how ever it could ruin their live sif they contract something else, mainly a baby. I dont know If your ready to be a grandma, but from the first time your son has relations you ahve to be ready in the case of a busted condom, make sur she is taking some sort of birth control pill, or patch. You can tell him a minimum age, but unless his respect for his parents outmatches his urges (unlikely in a teenager, even though you may be good parents, many teens resent good parents due to the fact that they want independence, good parents tend to be involved.
) I would say 15 thats an early enough age that he/she may listen to a parent, unless they are sort of pressured in, No is not always seen as an option amongst teens.