The two biggest factors in aging beef are:

• Size (weight)
• Fat cover.

Other contributing factors are:

• Age of the animal.
• How full the butchers locker is
• What temp the locker is kept at.

Size:

Basically it breaks down like this ... The bigger it is (providing there is enought fat cover) the longer it is able to age. A 900 lb (hanging weight) carcass is going to be able to age longer than a 400 lb carcass.

Fat Cover:

Fat cover means the amount of fat that is covering the outside of the carcass. The fatter the carcass is the longer it is able to age. A 1000 lb bull carcass is only going to be able to age 5-10 days where a 700 lb finished steer can go 14-21 days.

Below are 2 great examples of adequate fat cover. Both have a nice layer of fat covering the carcass, allowing for a maximum age time.
This is a 600 lb carcass produced By Rich Holmgren of Bear River City, UT. This carcass aged for 12 days and was a choice carcass.	This is a 750 lb carcass produced by John Ferry of Corinne, UT. This carcass aged for 15 days and was a choice carcass.

Age of the animal:

Typically a younger beef (18-30 months) is going to have better fat cover and weigh more than a 30+ month animal.

How full the butchers locker is:

Bacteria is what breaks down the muscle structure (aging). In a full locker the bacteria is able to easily "jump" from carcass to carcass. Being able to "jump" on a fresh carcass jumpstarts the aging process and makes it so the animal doesnt need to hang as long.

Also if a butchers locker is full he may not have the room to age your animal. Lockers are very expensive to run and keeping your animal more than 14 days costs the butcher money. Your animal is taking up room that another animal could be in.

What temperature the meat locker is kept at:

The majority of lockers are kept between 34f and 38f degrees. The lower the temperature the slower the bacteria works. A locker kept at 38f WILL age faster than one kept at 34f.

Final Thoughts ...

There are many factors that contribute to how long your beef can age. While a 21 day age is thought to be "the holy grail of aging", not every carcass can hang tha long. More information on aging is found on Dry & Wet Aging Beef.