You can guess the length from the tip of it's nose to the middle of the eye. I killed one that was 4 1/2 years old and measured about 8 1/2 inches
Sorry, but that's just not feasible. As a taxidermist, I measure the eye-nose one every deer, though it's not to the center of the eye. As a general rule, older deer do have larger heads, but the difference between a large 2.5 year old and a 5.5 year old can only be 1/4". There's no way you can see that until the deer is dead.
The fact is most people are guessing. Some guess better than others.