from wiki:
To qualify as a rookie in the National Hockey League, a player must not have played 25 regular season games or more in any single season. As of the 1990-91 NHL season, a player also must be 26 years old or younger to qualify as a rookie.[9][10] The National Hockey League awards the best rookie with the Calder Memorial Trophy, as voted upon by the Professional Hockey Writers' Association. In the NHL, rookies have special contract rules which limit how much a team can pay them as well as limiting the length of the contract. An NHL rookie contract is called an Entry Level contract and is typically limited to three years.[11]