My parents used to let me have a few sips of their wine when I was a kid, 6 or 7. I was drinking with friends at about 17, and then drank like a fish at university from 18-21, then not as much when I started a career, just on weekends, and even less so now I'm a father.
I don't think it made a difference. I thought beer as a kid was disgusting, and as an adult it tasted better. When I was a student, the price was more important than the taste. Now, I'd prefer something that tastes good, rather than how much it costs.
I don't get the 21 age limit for alcohol. Surely the majority of kids at Uni drink anyway, so they're just doing it illegally?