Fourteen used to be the age at which a child could be left to look after a younger child. I doubt that meant at night, probably only for a while during the day but I don't know. That seems reasonable.
At twelve, a child is deemed old enough to be left at home alone during the day.
I've no idea about being left in cars, I can't see any harm in a fairly big child being left for a very short while if mum or dad nip into a shop as long as the car is outside the shop and not likely to be nicked with the child inside.
They are guidelines only, as Rose says children mature at different ages. I imagine the rules were formed to prevent parents neglecting young kids but leaving, say, a ten year old alone while a parent, eg, goes to the post box or corner shop, doesn't seem unreasonable and I doubt the police would consider that a crime.
Older children may be a bit scared being left on their own for any length of time, even if they are mature in other ways.