I can have one glass of wine or a beer with no ill effects. Sometimes I'll have 2, but unless it's over a long period of time, the second one makes me drunk. Even one glass will make me wake up dehydrated and that affects my cycling a lot.
I love wine and have 1-3 glasses a week. I would have a glass every night if it didn't affect me so much. For awhile the sulfites did me in, but once I got my asthma under control, that went away. I didn't drink at all until I was out of college (too busy with other controlled substances), so I can't really compare how my reaction to alcohol has changed. I don't drink hard liquor, but I used to when I was younger.
Jewish people also (supposedly) have the gene that makes you not able to drink as much. I don't think I have it, I'm just small and it doesn't take much for me to feel the effects. But, one of my sons definitely has the "silly" gene from drinking. The other one (the marine) drinks way too much beer and shows no ill effects...