The bible does not talk about age in relation to sex. Depending on who you talk to, sex with a boy may be sinful based on two criteria:

1. It would be sex outside of marriage, which many Christians believe to be sinful.

2. It would be homosexual sex, which many Christians believe to be sinful.

If you subscribe to either of those then it will never be ok to have sex with this young man unless you marry him first (assuming you subscribe to #1 above, but not to #2).

If you don't subscribe to either of those views, then it becomes a lot more difficult to claim it is sinful.