Is it because that's when many graduate high school?
A lot of research indicates that females brains mature around 21 and males around 25.
IMO, 18-25 year old males shouldn't be able to legally purchase AR-15s, shouldn't probably be drinking alcohol and shouldn't be consuming cannabis. Should they be entering contracts, voting and enlisting in the military? Not sure. Should they be punished in the criminal justice system equally to an older person with a fully developed brain? Maybe a sub-adult category is warranted with some restrictions applied to 18-25.
Link: https://bigthink.com/neuropsych/adult-brain/