Reaching 21 was always associated with "key of the Door" but now I suppose 18 has somehow taken over. But the phrase was brought about by the fact that you had become an adult and as such could come and go as you pleased without parents consent. Thus, you became eligible to "the key of the door"
So why not treat them to a Happy 21st Birthday Personalised Mug.
The mug has the number 21, printed 9 times in a square on the front with the words Happy Birthday going around the square.
To make it special you can add your own personal message on the reverse.
A great personalised mug to be used daily.