Since we're talking about kissing, some of my kids (they're 8 of them) have come to me in their teens and asked, "How old do you have to be to kiss?", "When is kissing alright?", or "Is kissing wrong?" That's such a loaded question for a parent!
On the one hand you don't want to encourage it, like it's all good, go ahead, kiss whoever you want. Then on the other hand you don't want to discourage it, make them all freaked out and repressed as if it's wrong. And sometimes kids just want to find out what their parent's are going to say just so they can do the opposite. But you're not sure when they're going to use that tactic. See the problem? So I've always handled it this way - with the truth.
I tell them kissing is fun. Very fun. That I really haven't met too many people who are just starting out on the relationship road say that kissing is boring. I tell them it's exciting. I tell them it drives a mix of emotions. I tell them it's meant to lead us to greater intimacy. I tell them once you start you really don't want to stop. I tell them that it's very hard to go in reverse once you start. Lastly, I don't tell them when it's okay to kiss. I just say, "Put it off as long as possible." That's it. I put it back on them to decide. Risky I know. But to my surprise most of them have put it off much longer than... well never mind. My kids read my blog sometimes.