If you played for four seasons in Junior (Kesler, Spezza)
If you are 20 years old as of sept 17th I believe
Thats what I think
Now, I thought Kesler played for Ohio State. Now somebody in Canadian Juniors would have be of 20 years in hockey age, but I think if they really wanted to bring over a European before that age they could (see Prospal, Vinny) but they usually don't because it would hurt junior programs in foreign countries. I'm not sure Kesler was 20 in hockey age either. I say the 20 thing is for Canadian juniors. I also say teams are allowed 6 players with over 300 games professional experience not counting the goalies.
NCAA guys can play in the AHL anytime after being drafted.
CHL guys need to complete 4 years of Junior, or be 20. (Spezza)
Also CHL guys can report to AHL teams after their club team gets eliminated, this only happens when they are 19 or 20. For example, Tanguay played 5 games for Hershey as a 19 year old after Halifax was eliminated from the QMJHL playoffs.