The traditional Christian view is that sex is only for marriage. In that context the answer to your question is when the person is old enough to enter a permanent relationship. A lot of countries allow early marriage (under 18) if the parents consent, assuming that the parents know the young person well enough to answer that question.

More broadly I'm please for you that he wants to have you in his life. Why do assume that it's got a sexual content for him? You may have reasons for that belief, but it's a mistake that we can make in assuming that's a part of the boy's agenda when it isn't... Equally we can assume the opposite of course! Certainly clarity on these sorts of things can be very helpful; several teenagers - many decades ago - expressed a strong anti-gay opinion early in their friendship with me that removed all doubt. Recently a now growing friendship with a guy in his 20s got off to a problem start when my invite seemed to him to be a 'gay date', which wasn't my intention. Fortunately he was willing to use an email to ask for clarification - a useful distance to avoid embarrassment - and we got past that. Actually that is one of the burdens that the legitimation of gay sexuality has imposed; all otherwise non-sexual friendships are now subject to ambiguity.

I'm interested to see you think your AoA is moving up. That's certainly my experience, and certainly makes life less problematic!