If they were minors they have a plain old less than adult brain, less moral agency, and are far more easily manipulated.
That is why laws are different for minors and adults.
Yes the day before they turn 18 is very similar to the day after but you have to draw a line somewhere. Where would you draw it?
Where would you draw it?
Many young have good reasoning skills inmost part by good parenting and family style and most go on to do good deeds.