Mainly body, also face. Sway belly, shoulder size, roman nose etc. Antler size can work for first couple of years. Again, a lot of research on whitetail aging that translates to mule deer. If you watch the area from year to year, you will start to recognize deer and know their age from from history.
Cindy, I used to head up a GM organisation in Australia where there was, or still is, a rule that only stags of 6 years and older can be harvested, this allows for younger animals to bring through any good trophy potential. Easier said than done with so many new and not so new hunters coming through, most have a lot of trouble aging on the hoof.
Buster is pretty well onto it, as are the other posters.
For me and what I point out to 'our' hunters, is the first place to look at on an animal to start ascertaining an age is the pedicles. Are they long, or close to the skull, as in Nick's pic. Obviously if they are a long way off, the animal still has a lot of growing to do and there is a good chance he will grow more points.
Next, look at neck size and how he carries his head.
I have zero experience with Mulies, but antler growing is antler growing regardless of species.
There is a great book called 'Whitetail Racks', which is a great read for any trophy hunter, here is a link,
http://www.ebay.com/itm/Whitetail-R...d-Out-Brand-New-Hard-Cover-Book-/360786260994
Looking at antler size to age an animal is not very accurate, as in the scenario of being in an area with great genetics, you may have a 3 year old monster that truly will grow into a record breaker, someone says, 'look at the monster' simply because he has great antler growth, and harvests the animal. Yet the same animal, with age, would have grown into something really big.
Too answer your question, if you are pretty certain the 3 points in your managed area are mature animals, for me, I would pull the trigger.
Good luck with your management.