There are many reasons that butchers don't typically age meat these  days:

• First of all the cost of aged beef can be very high. Because of  the weight loss of aged beef, the price per pound can be pretty  outrageous. If you add in the time, storage space, refrigeration, and labor, that price just keeps moving up. 
• For aging to properly improve  the quality of a cut of meat, it should contain substantial marbling.  This means that there is fat evenly distributed throughout the meat. Only the highest grades have this kind of marbling (Choice and Prime).

The aging of beef is normally thought of as the time, in days, from  slaughter until the carcass is broken down into retail cuts. The  average industry time for aging beef before cutting the carcass into  retail cuts is about seven days.  We typically hang most beef  for 10 to 14 days.

Cooked, un aged beef has been described as "metallic" and lacking in  typical beef flavor.  True beef flavor is fully developed after about  11 days of aging. The aged beef flavor increases with increasing aging  time.  However, the longer the beef hangs the more weight you lose.

Aging also increases tenderness. It has been shown that during the  aging process certain changes take place in portions of the structure  of collagen and muscle fibers. Currently, it is thought that  enzymatic-caused changes in the structure of muscle fibers are largely  responsible for the increase in tenderness. It is known that  tenderness decreases immediately after slaughter while rigor mortis  takes place (taking 6 to 12 hours to complete); then tenderness  increases gradually. Tenderness continues to increase up to 11 days,  after which there is no increase in tenderness.

The amount of time that it takes to get a 10 to 14 day age can also  vary.  If there is only one beef in a cooler the enzyme and bacteria  growth is slower and the beef may hang for 16 days to get the  equivalent aging of 10 days.  And if the cooler is full there is going  to be more enzymes and bacteria present.  So the beef may only need to  hang 8 days to get a 12 day age. It is necessary that a experienced  butcher closely watch the hanging carcass. During the aging process, one can also expect a loss of weight of the  product. Because the lean (exclusive of trimable fat and bone) is  approximately 70 percent water, it's easy to see why there is a weight  loss. The weight loss is caused by dehydration of the lean and fat.  The weight loss occasionally occurs at tremendous proportions  depending on relative humidity, amount of air flow and temperature of  the aging cooler. During chilling of the hot carcass immediately after  slaughter, the carcass will lose 2 to 3 percent of its weight due to moisture loss. Aging the carcass beyond this time will result in  additional tissue shrinkage of 1 to 1.5 percent per day for each seven days.  Hamburger cows or "grinders" should be cut within 3 days of slaughter to minimize the weight lost per day.

Carcasses with a thin external fat cover will lose more moisture than  carcasses with a heavy fat cover. One study observed an 18 percent  trim and shrink loss from loins aged 14 days in a 36 degrees F cooler. Also remember that fat protects the meat from dehydration. Therefore,  if you are aging a beef carcass with very little fat, you can expect a  higher weight loss during the aging process than would occur normally  with a fatter carcass. Maintaining the aging cooler at 85 percent  relative humidity will keep weight losses down during prolonged aging.  Carcasses with little external fat are more likely to pickup  undesirable cooler odors and should thus be aged no more than five days.

Some studies have shown that there are many benefits of hanging a beef  for up to 21 days.  These benefits greatly decrease after 14 days.   With the new e coli and salmonella concerns we don't feel the benefits  out way the risks of hanging a beef for 21 days.