A persons cognitive capacity to understand certain situations and act accordingly - whether it be unplanned pregnancy or something a lot worse - is not based on whether they have been told that it's right or wrong.
A young teen (of 13 or so, as you mentioned) may lack the emotional maturity to control impulsive behaviour, or lack the emotional maturity to understand risk. Another young teen may be the opposite. There is already evidence to show that genders mature at different rates. Maturity is learnt, which is why people state that you 'mature with age'.
You can't determine "maturity" as a concept by one set age...you just can't.