Some practical uses of “a natural lifetime”

Then Wendy began to see that one didn't stay at two for the rest of one's life. Indeed two is the beginning of the end. The end is being grown-up. Once you get to twenty one or so, you can never be ungrown-up again. But Mrs. Darling did not tell this to Wendy. Between two and twenty one, there was lots of time for her to find out.