Yeah alcohol does seem to be the predominant gateway
I wonder though, the reasoning behind it may vary a lot.
My parents for example, from about the age of 6 onwards, always allowed me to have a small glass of beer, shandy or wine with my dinner if I wanted. When I was first offered I of course accepted, but soon learned I didn't like the taste. They still always offered me a drink with my meal, but I became accustomed to saying 'no' and opting for water or squash.
When I was a little older I started drinking small amounts at home, by the time I was 14 and my friends were getting smashed, I had a pretty good rein on my drinking habits. Once or twice I was excessive, but never to the extent of some of my friends.
My parents always told me the reasoning behind this was that they believed prohibiting something particularly for young people makes it more appealing, and they firmly believed I was my own person developing my own understanding of my body and thus capable of making my own decisions. They told me that in parts of europe alcohol isn't demonised so much for children, and that they have a much smaller binge drinking culture.
Although like Tharcila points out, If alcohol had been prohibited, none of the above would of been possible and I'd probably be a reckless alcoholic by now.