I reckon it should be 16 for indictable offences/felonies and 21 for summary offences/misdemeanors.
at 16 years old you are fully aware of the severity of your actions if you break into someones house, beat the shit out of them, and steal their car LOL but i think the propensity to do little shit like vandalism/petty theft/smoking weed is pretty similiar from ages 16-21 and I think as a uni student/apprentice/high school grad who lives at home, doing things like that at that age shouldn't fuck up your whole life forever because you are still young and not yet settled i guess. so there probably should be a kid period 0-15, young adult 16-21, and adult 21+ category. after spending alot of time in america where alot of things are 21+ I do understand where they come from with that age being the barrier to true adulthood, the difference between a 17 year old and a 19 year old is nothing compared to the difference between a 15 year old and 17 year old. from an australian perspective 18 seems fine to us because at 18 you can smoke drink and gamble and it has always been that way, but after studying over there and seeing their point of view I totally get why from an an objective point of view or the pov of a 40 year old, why 21 is a better age to allow people to smoke drink gamble etc. I knew alot of kids who turned 18 in year 12 and would just go to the pub every weekend smoking and drinking beer just because they could even though in retrospect they really shouldn'tve been allowed to at that point in their life based on their maturity level. but i really think once you get to 20 21 you are alot more well adjusted and not going to childishly abuse new privileges. i dunno i am kinda sleep deprived atm and gonna go to sleep goodnight everyone