Good question.
I think the correct answer is that then (13) is when he first started out. He reached his “high” when he was 15, that’s when he want on an aveira rampage.
My question is it’s quoted that the reason or excuse he went totally of (by 15) is because he saw Avraham Avinu – the biggest tzaddik – being niftar. So he went out and said ??? ??? ???? ????. Here we’re told that Avraham Avinu was niftar because of him. So if he wouldn’t have passed away Eisav wouldn’t have gone out ?????? ???. So then he wouldn’t have to be niftar. So then He wouldn’t have gone astray. So then…