when did rudolph become a reindeer

Rudolph has always been a reindeer. He was born at the North Pole just like the rest of us. But, unlike us, he was born with a red nose. Santa knew from the moment he saw Rudolph that he would be special, and he was right. Rudolph's nose has the power to light up the way through the darkest of nights, and he has become an important member of the reindeer team, helping us to guide Santa's sleigh safely through the skies.