The kind of drinking I’m talking about falls directly between a couple of glasses of wine and falling down, puking drunk. It’s definitely more than a couple of glasses of wine (more like a bunch of bottles between six people, plus assorted beers, coolers, and hard liquor).
DianaG - I don’t drink, and I have fun all the time. It can be done. 
I can see why you wouldn’t have a good time at those gatherings, featherlou,even though it doesn’t seem like a cause for concern. If every family gathering I went to involved, for example, lots and lots of discussion about football, I wouldn’t have a great time. Nothing wrong with football, it’s just not my cup of tea.
Would it be possible for you to host a family gathering without alcohol? Not by making a big hairy deal about it, but by getting together for a pancake breakfast or some kind of outing at a public park that doesn’t allow alcohol? That way, you’d get to hang out with your family, but it would cut out the booze factor.
FWIW my uncles in their 70s still drink to excess.
I often have fun without drinking as well. There are lots of different kinds of fun. I just don’t understand why a bunch of people having a sort of fun that you don’t particularly care for, that isn’t, for instance, kicking puppies, makes you wonder what’s wrong with them?
In my sphere of friends it was like a light switch when we hit 30. We just stopped drinking in any quantity. Drunk and or sick from being drunk just isn’t any fun.
I was having dinner with a friend of mine a couple of months ago and we both had 2 margaritas. We didn’t realize it until it was too late but they had WAAAAY too much alcohol in them. We were pissed.
I can have fun without drinking too. But not at a drinking party. 
My answer to the question posed in the title is “the same amount which is too much when you are younger”–although it’s not unheard of for older people to be taking prescription medicine which should not mix with alcohol (OK, so the same can also be said of younger people).
Still, while there are plenty of reasons why someone who is no longer a callow youth might choose to watch their alcohol consumption more than the callow youth does, if the only repercussions are hangovers the next day, I think the people are drinking in a manner that I will try not to judge them for. (As someone who almost never touches alcohol, I have a hard time judging what constitutes responsible social drinking, and generally try to give people the benefit of the doubt. But my fingers balked at typing the notion that routinely drinking to the point of hangovers constituted responsible drinking).
One thing that I think convinces many folk to reduce their drinking at family events is that it sets a pretty questionable example for kids - especially as they reach an age when you want them to be making responsible decisions about drinking, drugs, driving, sex, etc.
Also, I found as each decade passed the hangovers seemed to hit harder and harder. And as it became clear that I had fewer and fewer years ahead of me the idea of simply pissing away days recovering from hangovers didn’t appeal to me quite so much. Not to mention dealing with kids the next day when hungover.
I remember praying to the porcelain god on my 40th b-day, and thinking that would be the last time ever. It wasn’t, but I quit altogether less than 5 years later.
There’s nothing wrong with family members getting a good buzz on when together. But that occurs far before you start to slur and stumble.
I think you’ve misunderstood the question - I’m not wondering what’s wrong with them, I’m wondering IF there’s something wrong with them. Most people here are saying that there isn’t anything wrong with it, so I am internalizing this new information.
Dinsdale, I’d love to be a fly on the wall watching my sister try to explain to her two daughters (13 and 10) why they shouldn’t drink until they get drunk when mom and two of her sisters do it a couple six times a year. 