To clarify I went in November 05 (I was 18 then, am 19 now) and just came back from Thailand for a second time last month. I had no problems with bars, clubs etc. - both in Patong and Bangkok. If you take a lady with you, often they'll ask her for ID but I didn't get asked. Granted, I'm 6'5 and probably look a bit older than my age, but nonetheless I think if you're on holiday and having a good time the general attitude is always positive.
Regarding age, I certainly wouldn't worry about it.. and to answer your question - Do bears shit in the woods? Yes, they'll love you. Mind you - with the BGs it's still all about the money so I wouldn't go there thinking "Yeah, I'm young, handsome.. maybe I'll see if I can get out of paying for it" cause you'll be in for a lonely night or 3.l