from CapFriendly,

When a player is reassigned from the NHL to the AHL, they must pass through waivers (unless exempt). Waivers allow any of the other teams to claim the rights of the player.
This tool determines when a player will no-longer be waiver exempt.

Age is defined by the following:
If a player turns 18 between January 1 and September 15 on the year of the entry draft, they are considered 18
If a player turns 19 before December 31 of the year on the calendar year of the entry draft, they are considered 19
If a player turns 20 before December 31 of the year on the calendar year of the entry draft, they are considered 20, and so on.

continue to search a list of exempt and non-exempt players...