There are some basic body "shapes" that help identify a buck's age, but you kind of have to "read between the body size." In essence, you have to ignore the over-all body size and look for specific characteristics.
Just remember that body shape is going to change dramatically from the late summer and opening of bow season to November (rut). Even older bucks look more "meek" in late summer. There is also a difference in a buck’s body shape and size in different regions of North America. For example a southern United States buck compared to a Canadian buck of the same age looks much smaller and dainty. For comparison see the two pictures below side by side of a Saskatchewan buck (top) and an Alabaman buck (bottom) of about the same age class.
For yearling bucks, even in Canada where their body weight can be high, yearlings still have the look of a large doe with antlers.
2 1/2 Year old Buck
2 1/2-year-old bucks are more muscular in chest, but not as much more in hams than a yearling, but they will have a large neck near the rut. In fact, the easiest way to identify a yearling from a 2 1/2 is the neck. If it has a big thick neck, it is not a yearling.
3 1/2 Year old Buck
3 1/2-year-old bucks look very much like thoroughbred race-horses. Very trim waist, but shoulders and hams are very muscular.
4 1/2 Year old Buck
4 1/2-year-old bucks are really at the peak of their game. Muscular in hams and shoulders, but belly is starting to get big, with some sag. The neck of a 4 1/2 is so large it kind of blends straight into the chest with little definition between the two, as in no destinct line between the end of the neck and the beginning of the chest.
5 1/2 Years and older Bucks
5 1/2+ year-old bucks have a lot of belly sag and the back starts to become swayed. Neck is huge with no definition between neck and chest.