When I first heard that 'aged' meat was supposedly preferred over fresh meat, I thought it was a joke! But supposedly, hanging it out to get older and aged increases flavor and the natural enzymes make the meat more tender. Dry aging only works with fatty marbled meat but wet aging is more flexible and faster. This brings all kind of things to mind. First, it means apparently that you can hang a slab of fatty meat out for weeks and it is only supposed to get better! They even say that in dry aging, a 'crust of mold' will sometimes form that will actually increase the taste and tenderness even more! Remember that next time you go to a fine steak house!
However, these days, they usually do wet aging which only takes a few days and minimizes weight loss in the meat. Still, could aged meat really be better for us than fresh meat? If not, then why do we think it tastes better? To me, this actually does make sense, though, that humans would develop the ability and even enjoyment of saving meat and eating it even when it is old. That way, the meat from a big hunt could be eaten over the course of weeks instead of wasted. This also means a big hunt could keep a small tribe plush in edible food for a long time, even in warmer climes before the advent of refrigeration. It also puts an interesting perspective in the advice to not even let meat sit out of the fridge for even 15 minutes!
I don't really think aged meat is healthier. It tastes better because the more of the tough connective tissue is broken down and water loss concentrates the flavours of the meat. This is due more to the natural enzymes in the meat rather than mold or bacteria, these also begin degrading the proteins and fats producing more aromatic flavours.
Aging also is only needed on meat from older animals like beef or from tougher game meats like pheasant. As it says in the link patrik posted, game meat like pheasant really needs to be aged to release its flavour. There is no real benefit from aging young meats like pork, lamb or chicken as these are usually slaughtered when the meat is still tender.
Aging meat is not the same as the decomposition produced by bacteria. Aging works at cool temperatures that suppress the growth of most bacteria, unless it is winter this it is done in a fridge. If you tried it at warm temperatures the result would soon be stinking rotten meat. Molds and bacteria that will grow on aging meat in cold conditions are usually environmental in origin and not be carried by the animal, such molds spores and bacteria are everywhere in the environment.
There are some traditions where meat is fermented by bacteria. Hákarl is an example of fermented skark from Iceland. It is apparently an aquired taste.
Hákarl is traditionally prepared by gutting and beheading a Greenland or basking shark and placing it in a shallow hole dug in gravelly-sand, with the now-cleaned cavity resting on a slight hill. The shark is then covered with sand and gravel, and stones are then placed on top of the sand in order to press the shark. The fluids from the shark are in this way pressed out of the body. The shark ferments for 6–12 weeks depending on the season in this fashion.
Following this curing period, the shark is then cut into strips and hung to dry for several months. During this drying period a brown crust will develop, which is removed prior to cutting the shark into small pieces and serving.
I think that there would be some factors that would affect whether or not aged meat is a good thing. The ones that come to mind:
-What is the microbial activity where the meat will be aged? (For example, a friend of mine has been unable to pickle foods in her home, using the exact same recipe and methods that I do. She recently discovered that she has a horrendous mold problem all throughout her house, and this is the like reason.)
-How did the animal live and die? What an animal eats and how it is slaughtered and cleaned affects the amount and type of bacteria in the meat, for better or worse.
In general, I feel that we are breeding unhealthy (for humans) microbes, in our bodies and our world. I can't help but think that the overuse of antibiotics in recent years is making it harder for the good bugs to thrive, and that our health is suffering for it. But this is a tangent...
I can remember only one kind of aged meat traditionally done in old Polish cuisine - wild caught partridge. I remember reading how the smell was pretty putrid and horrible, but the meat was delicacy. I have never had a chance to try, though, it's a bit lost art.
We have kind of opposite signals from it. On one hand the taste might be good which could indicate it was good for us, on the other hand horrible smell which should be a warning.
Drying and smoking is a very popular method and was used for centuries (along salting) to preserve the meat. I think this method helped stopping the process of rotting and decomposing which might be far from healthy.
And yeah, today we have so many additional factors like antibiotics, pollutant and unhealthy diets (Even grass-fed - who knows what chemicals were spread over the land before it became a pasture?) that it is very difficult to assess the benefits of diets/methods that might have been used thousands of years ago.