Most deer are fairly predictable as to their age by looking at antler growth. In my hunting regulations book it has a chart to show how to judge by this. It has proved fairly accurate for me. This will change from region to region however and so you should just learn to judge the deer in your area. I judge them by their antlers and if I end up shooting a giant young buck by accident I don't care in the least. Anyways, judging age through antlers can work but you have to remember that it's only an estimate and not certain. Deer are predictable except when they get bad or superior genes. Learn to tell age by their body size and other things too. Try to estimate the age of every deer you see and soon you'll be good at it.