Further to the previous answers:
It was traditional, in England for the older son or daughter, upon reaching 21, to receive a key to the front door of their parents home, if they were at that time still living there. It was the key to adulthood, but they were often also given the privilege to come and go as they pleased, at what time they liked.
As a child in the fifties I was often told in answer to a question, "I'll tell you when you're 21." Being twenty-one was considered being grown up.
Everyone up until the age of twenty was still, even if they had begun work, considered a child and subject to their parents authority. In 1968, my 18 year old boyfriend was grounded by his father for coming in at 1.00am on New Year's Eve. Nine or ten o'clock was considered the latest one should be out with a girlfriend in those days.