You can actually usually get pretty close at guessing a deer's age on the hoof using body characteristics.
There are some regional differences, northern deer develop larger bodies faster than southern deer so that makes a difference.
The best time to accurately age a buck on the hoof is during the rut. The amount of staining on their tarsal glands, how much swelling of the neck, etc. make aging easier than in the summer.
Here is a good article on aging deer in the southeast -
http://msucares.com/pubs/publications/p2206.pdf