I do not use the "color of the coat" for an ageing factor. Deer, like people, can have different hair color variations. If the deer is alive, look at the head first. Is it long from nose to back of head? Nose to eye is a good indicator also. Then look at the body. Chest to rump. Is it long and deep or thick? Is the belly starting to sag? If other deer are around, is the one you're looking at tall? Sometimes it is hard to tell on single does. If it is a buck, antler size and mass is a good indicator. Especially the mass. Mass comes with age. Does the buck have a blocky head with a roman nose? Also, is the bucks neck practically the same size as the body where they join? These are all things you should look at when ageing deer. If the deer is dead, look at the size and shape of the feet and inspect the teeth.

BTW, for judging a bucks inside spread in the areas that I hunt, I use this method. 2 1/2 yos have an ear to ear spread of 16". 3 1/2s are about 17". 4 1/2 and up can be 18 ".
Good luck.