It's hard to imagine, but when DS is in year 12 he will be 18 at the beginning of the year so I hope he doesn't get carried away with being "legal" and forget about school!
Would not worry too much, there are kids that do and kids that don't. I turned 18 in year 12 and it did not even cross my mind to go out drinking. Those that were drinking were already doing so when they were 17. So age does not stop them if they want to.

DD2's birthday is 28th April, so if she was born 3 days later, we would have no choice. I am leaning towards sending her to Kinder the year she turns 5.