Deer and Deer hunting has a good article this month as well. One thing I thought interesting was at 4 1/2, they said the mass circumference should be about the same as the diameter of the eye. Not that I'm going to pass a 3 1/2 because he's too young, but thought that was a measurement I'd never heard before.
Here are my thoughts on it.
1 1/2 still have long legs, no belly and an arched back. Rack, unless they are a freak, will be nothing too special.
2 1/2, you start to fill out the chest a little bigger, still no belly though and the back is starting to straighten out.
3 1/2, you start to get the back sway, notice a belly appearing but will still have the 'Race-horse' look. The bigger head/nose will be visible and the chest is really starting to develop.