I know penis size is determined by genetics, but testosterone also plays a role during adolescence, yes? masturbation depletes testosterone from the testes. therefore, masturbation during adolescence can reduce possible penis size. is this correct?

No effect on size: People do not gain nor lose testosterone due to masturbation. Masturbating more often won't cause a low-testosterone problem. A lack of masturbation doesn't cause a high-testosterone problem. One's body does not know how many people are engaged in the sexual activity. Whether having sex 6 times a day with 1 other person in the room or no other person present, the body DOES NOT know the difference.