OK, I'm a hunter, and I am in the woods almost every day. I seem Moose, and Caribou all the time, and I can tell a young animal from and old one at just a glance. Some people say it is from experience, and I have to agree. I'll use Caribou as an example. I often take new people out to look at or to hunt Caribou. All Caribou have antlers after the first year. Cows have spendly small antlers that look somewhat like a yearling Bull, but the body is differant. I can tell at a glance a young bull from a cow by the body, but a new person can not make that call. They look the same to them, till I point out the differance on body mass. How the little bull has a larger chest area, and lighter rear quarters than a mature cow.
Another point: A yearling will be jumping around, playing. When they move while eating they will make a lot of wasted movements by dancing around from one spot to another. While a marute animal will not waste the movements. Another is body mass, yearlings will not weigh as much as a mature animal. Certain parts of the body have not filled out like a mature animal has. Their coats are smooth and unblimished from accidents or fighting for domanance.