When they begin imagining how beautiful sex would be with one another. This is the easiest of all questions to answer.
There's far too much pointless hype in the world nowadays... people worrying about what their parents might think if they were to find out, whether or not they should wait until marriage.
Honestly, if two people love each other and want to experience the most amazing gift love has to offer, why think it through for a second? We're all here to love, to feel. Making love to the man or woman that you love, or are super attracted to, is probably the best feeling in the world on at least 10 different levels. We're here to love and to feel.
So, put simply, two people should make love to each other when those thoughts begin to cross their minds. Enough said.