You need to pick an age as the “magical day” anyway. Not really a good argument
The human mind doesn’t even really fully mature until your mid 20s. A 15 year old still has a good full decade until full maturity, and they are notorious for making impulsive decisions without realizing the consequences of their actions.
What he did was wrong and he deserves punishment, but ruining his life too for being a dumb teenager does nothing for the unimaginable harm caused to this girl, it just makes more victims.
I don’t know what the right answer is, but I can tell you the wrong answer is to ruin a teenagers life over a stupid act when that isn’t going to solve anything.