I think it depends on the child, however not earlier than 16. I was immature, and it was, in retrospect a GREAT decision for my dad to have made. but that is only because I know of the ANGST it saved me. My view is very conservitive and I wouldn't force it on anyone but here is what our family belives.My husband and I are youth pastors and we talk to youth all the time about this.
I have also always believed that maturity and sense of responsibility means far more than age.
My guess is it depends on the child versus the age. For the moment I know who the kids are, but I too was a teen so I know anything can change.
That said - I only have one so I don't have the younger ones saying "but she got to at 14". The main thing I do is try to limit the amount of time she is there and the frequency.
When dating is delay until 16, the likelihood of her having sex is decreased significantly.
When dating is not permitted until 17, the liklihood of her having sex is much less, I think only 20 percent if I remember correctly. I honestly think that if she wants to date that badly, she will find a way. She is probably not like this, and I don't mean to sound rude or anything, just that if any child really wants to do something, they will usually find a way. Have you talked with her honestly about what you and your husband's concerns are?