My parents used to tell me, at 18 your an adult and some of my friends received luggage on their 18th birthday, meaning; "Its time you were on your own".
Today, the age has moved to 24. Higher education milestones being delayed plus biological arguments , including that the body continues to develop. For example, the brain continues to mature beyond the age of 20, working faster and more efficiently.
And many people's wisdom teeth don't come through until the age of 25 (So what!).
I thought it was because we have become helicopter parents. Here's the article.