When I was in school, I often drank while playing, but never more than a few beers, and that was fine. Fast forward to one of my first gigs after about nine years off, when I started drinking a pint on stage. I barely drank at all generally, and about half way through the pint, I noticed my timing had taken a serious turn for the worse. I switched basses to "reboot," and the rest of the gig was fine, but I've never consumed alcohol during practice or gigs since then, and I don't plan to. I might have a couple to come down after a set, but that's about it.
Yep, things change when we grow older. Our bodies can't metabolize alcohol (or any foreign substance) as easily.
My current band plays one gigantic set...3 hours long. So, this keeps me from drinking between "sets" like I used to in previous bands. So, I take a few slugs on my beer between songs just for thirst, but I never lose sobriety or motor control when playing live.
The other guys in my band -- ages mid-30's to about 50 -- drink heavily during practice, and I don't understand how they manage. Thankfully they don't do this on stage.
That's good they don't do it live. I think the reasons why some people have higher tolerances are manifold: age, race, food consumption, body weight, build, body fat, metabolism (active/lethargic). Race is highly significant. My stepmother is Chinese and it's no secret that she, her family, and all her friends have very low tolerance for alcohol. Halfway through a glass of wine and she starts flushing red, getting giggly, etc.
I'm primarily of German & Irish decent and have a pretty high tolerance...yet I'm 1/8 Native American, so I'm not as resilient as I would like to think.
All in all, personal practice time should be sober...it's when your brain is building neural pathways...alcohol impairs this process and you'll spin your wheels.
Live performance should be sober.
Rehearsal for me is when I like to "let my hair down" - it's just one of my weaknesses I guess. I probably shouldn't drink anything during rehearsal either...but I do. Of course, we're always learning...so when playing at any time, we should regard it as a brain-building exercise and forgo the drinks.