only way to guess age is spur legnth.

bumps are a jake(1st season bird)
1/2-3/4 is a 2yo
1+is a 3yo
higher than 1.5 or so it gets tricky..

thats how i usually go...some guys say 1inchers are 2yo..i dont think so. atleast not in PA from what i have seen....i can almost tell by how they act sometimes..