I stopped drinking about two years ago and I'm 31. I realized I just didn't like it like I used to. Don't like the way my body feels even after two beers. I used to love drinking, but I think for awhile I was drinking solely out of habit and social reasons. I did go through a big caft beer period before letting it go.
Pretty much just smoke weed now. Booze is really expensive if you're a regular drinker, especially in public, and it's difficult to function drunk. I can make a bag of weed go much farther, function fairly normally stoned, and never have to worry about being hung over or sick.
There's a new arcade bar here in the city, our first one, and I'm so glad I don't drink. All the games are on freeplay (except pinball, which is still cheap) and there's no cover, but the beer is $6.50 a pint or more to compensate. I can have a great time for free (or a few bucks shooting pins) while my friends are dropping $20-40, easily.