I think it happens when you don't have any doubts. You just feel such a strong connection with the person you're with, you feel really confortable and you want to take the next step.
But I agree with the girls above, don't even pressure yourself to do it, that's the worst. Take your time, make sure you're with someone who cares about you :)
There is never right time no one gonna tell you that today is your right time or when you turn 18 or anything like that . You have to be confident about it and with a person you want to be with . Never rush or force yourself cuz the you will have to face guilt . You just have to feel its right . Stay safe.