Key pendants are customarily given as a present on the 21st birthday. This tradition dates from the times when at twenty-one, you were considered old enough to be a key-holder to your family’s home, and thus hold a symbolical 'senior' position in the family. It's an old tradition that, some think, has long since become irrelevant and obsolete, but the symbolism of the key on the 21st birthday lives on. Both 21st birthday and Graduations are considered to be a "new beginning" occasion and key pendants are popular presents for both.
Keys have always been endowed with a certain sort of magic. It’s believed they have the power to reveal things that were unknown or obscured before. In Far East cultures, keys are regarded as a "Good Luck" presents and a bearer of a good fortune ahead. It was believed that wearing a Key as a Pendant will open physical, intellectual, or spiritual barriers and give access, or attract riches of all nature.