It all hinges on which league you were drafted from.
If a european born player comes to the CHL and is drafted out of the CHL than he must abide by the CHL rules, which state that you cant play in the AHL until you have completed 4 years in the CHL, or your team has been eliminated from the playoffs, or you are a 20 year old. Being a 20 year old in the 2005/2006 season means you were born in 1985.
Yandle was drafted out of Cushing Academy, US High School/Prep so he is eligible to play in the AHL at any age after his team drafted him, similar to what Chris Bourque did this season.