The french are internationally known for having poor quality beef. But that’s sort a non-sequitor.
“Exposure darkened” isn’t what you should be looking for, and exposure won’t darken meat. You leave an individual cut out the oxygen will lighten it up turning it pink. If it’s out too long it’ll oxidize and turn grey then poopy brown, and develop some nasty off flavors. The darker cuts you see in the case are usually just things that were cut more recently, a good butcher will cut to order though. Meat sitting out cut in a case is getting worse not better.
When we talk about aging we’re talking about what happens before it hits the case or shrink wrapped foam tray. And there’s kinda 2 things to look at. Fresh meat for eating now. And dry aged beef.
So for fresh meat two strategies. The traditional way, and modern wet aging.
In either case a dead animal goes into rigor. It will be more difficult to break up during that time. And it will be tough as hell and taste like licking a battery. So the meat has to be stored, preferably as a whole carcass, until that goes away. Then you can eat it.
Traditionally you do this by hanging in open air. By doing so you lose a lot of moisture, concentrate the flavors. You really only need to do this till rigor subsides, but there are benefits from doing it longer. Thing is you are fighting spoilage the whole time, and the longer you push it the more chance you’ll lose the meat. 1-2 weeks is the usual amount of time. You hear a lot about enzymes and flavor changes. But most of what happens here is moisture loss, and rigor going away. And honestly really fresh meat, right past rigor is really nice. Even if I’d say properly hung meat is better.
Modern meat packers want to do this faster. So they break down animals immediately, before or during rigor. Shrink wrap the primals so that the “aging” (ie rigor giving it up) can happen during shipping. There’s no moisture loss. It’s not bad per se, it just doesn’t bring anything additional to the party. Except shelf life. The vast majority of meat you run into is aged this way, even at high end butchers. And it is the only thing you will find at super markets. As it is pretty much the only way meat is processed for normal retail sale in most developed countries.
You can’t really do the long hanging thing with individual cuts. Though you can do it with primals. People who don’t have access to a walk in fridge often butcher deer rapidly and park the cuts in the fridge till rigor goes away. Though it’s always preferable to hang it, often in a shed or a garage if it’s cold enough out. My uncle used to hang his ducks in his mud room (2 weeks!).
Dry aging of beef is a different thing entirely, it’s a step that takes place after one of the other two. It’s basically mild fermentation, the outside of the meat is allowed to desiccate. Sealing off the interior, then mold and bacteria begin to break down the meat. Creating all sorts of fun weirdness. This does tend to be darker than other meat, mostly do to heavy moisture loss. But you can not do it to individual cuts, all cuts, or a whole carcass. It’s a treatment for specific primals (usually steak cuts).
Traditional butchers dealing with whole carcasses will hang the carcasses for a week or two. But those are few and far between these days. Even in Europe. Kind of why going direct to a farm is fun. Even if they don’t do it as a matter of course, they’ll often do it by request. Or send you to a butcher who can (and sportsman’s butchers tend to be game for it too). My pig guy likes to hang them 2 weeks, but if you’re in a rush it only takes 4 days for rigor to go away. Apparently you can pay him to hang and butcher a deer for you.
Burger’s got a lot of surface area. It’ll oxidize badly, fast. So it’s always best fresh ground.