Kidushin 30a #1: It is from 16 until 22 (he is a youth, and will accept your training).

Kidushin 30a #2: It is from 18 until 24.

Rashi: According to how you train him, good or bad [he will not veer, even when he grows old].

R. Yonah: Train a youth in good Midos and proper conduct; according to the path that he is standing on. If you begin to teach him and accustom him to apt matters that his nature is close to accepting them. And so accustom him from matter to matter and from path to path, according to what his intellect can accept. One does not reach perfection of Midos at once, rather, slowly, via his intellect. What is close to [his] nature, do not distance from him.

Malbim: There are two commands. (a) Train a youth. From his youth, accustom him to perfection in mindsets, actions and Midos. The habit of his youth will make a lasting impression in his Nefesh, and not depart in his old age. Training in his maturity will not make a lasting impression in his Nefesh; it will depart in his old age. (b) The training should be Al Pi Darko. Everyone is proper for different matters, based on his nature. Some have a sharp mind. Some have straight intellect, but not sharp. One must teach them according to the preparation in them. Some are prepared for a particular craft, and a special Midah, and receive it easily. One can recognize this in a youth from his desire. He himself strives for a particular matter. If one trains him for what he is prepared, it will not depart in his old age, but not if he trains him for what is outside his nature.

R. Yonah: During youth it is easy to straighten nature, fix Midos and accept habit. Also when he grows old he will not veer from it, like they say 'habit rules over all.' In old age it is hard to change nature, since he is used to it in practice. He needs great exertion to reverse it from its source. Therefore we are obligated to rebuke a Na'ar. His power of intellect is not complete. He has nature, and does not have rebuke from his own Nefesh. If he will get used to conducting like his nature, when his intellect is complete he will weary to change his habit and fix his nature. Therefore, his father and mother ¹ are commanded to help him with their intellect and fix his nature in youth, when it can be fixed more. A Mashal for this was brought from a branch (when it is young, thin and soft, one can bend it to the proper place, but not after it is old, thick and hard). This is put next to "Tzinim Pachim b'Derech Ikesh"; one must teach a youth that from the straight path one reaches perfection.