What is the Torah source for 13 and 12 being the ages to become chaiav on Mitzvot?

The Torah source for thirteen years old being the age of bar mitzvah comes from Parashat Vayishlach in Bereishit. When Shimon and Levi went to take revenge against Chamor, who raped their sister Dina, the pasuk says "ish b'charbo" - each man with his sword in his hand. The Midrash teaches us that each one was considered an "ish," a man. Levi was thirteen years old - he was the younger one. This shows us that at least thirteen years old is already considered a man according to the Torah. That's where the term bar mitzvah at thirteen years old comes from. For twelve years old for a girl, this is halacha l'Moshe miSinai - a law given to Moshe at Sinai as part of the Oral Torah. The age of thirteen for boys and twelve for girls being the age of obligation in mitzvot is transmitted through our mesorah, our tradition from Sinai. You need to understand something very important: even though a boy becomes obligated at thirteen and a girl at twelve, you don't start training them at that age. That's too late. You must train your children to keep mitzvot from when they're very young - even from infancy. By the time they reach bar or bat mitzvah age, keeping Torah and mitzvot should already be natural to them, part of who they are. If you wait until they're twelve or thirteen to start, you've already failed.