If you couldn't have bought alcohol until you were 21 you would simply have delayed going out and "getting really smashed" by a couple of years, not avoided it. You don't hit 21 and suddenly gain enlightenment.
For me, the best solution is to allow children moderate amounts of alcohol whilst they are growing up, get them used it to. My parents allowed me small amounts of lager and cider occasionally with meals from being really young and it took away the whole fascination of it. I didn't hit 18 and suddenly start going on on mass benders and getting blasted, I carried on drinking reasonably in the same way I had been doing for years. Plus, as I was used to it, alcohol didn't affect me in the same way as it did some of my friends who were out of their tree after a couple of pints to start with.