It does depend on the teenager. I was 16 doing the leaving, 17 by the time I started college. If I wasn't living at home should my parents have forbidden me from going to college in another part of the country cos I'd have to rent? What about a 17 year old mother, they do exist. She can look after her own child, even be married (possible with court exemption order?), but can't be left on her own for a night without her parents? Bet her husband would love that.
My parents went away last summer and left my 17 year old brother at home. Granted I only live 10 mins away and was instructed to check in on him every day and he'd rather spend the week at parties in other people's houses than throw one himself. Plus this was the first summer they left him alone, at 15 and 16 my sister moved back in, just to sleep, left him to it during the day.
At 14 I was babysitting for plenty of neighbours and minding the above brother who was 2. So I really don't think it's as cut and dried as one day it's irresponsible but if they turn 18 the next day it's not. Noone grows up over night.