Sex is about the spiritual and conscious connection with another individual. Time doesn't exist its just a creation to separate moments. So who cares how old they are if you're having fun, because the passion of being in a moment with another individual is all that matters, or one day you'll wake up and realise you're alone because nobody can stand you.
^ That's a beautiful theory to an individual, but unfortunately it's just not true in modern day context and I don't think it was true much in the past either.