There are a number of connotations stirred up when people think of the above phrase.
Some folks take it as a reference to becoming physically older, reaching puberty, gaining years in a physical sense.
I see it as losing hope. Losing your innocence. When you’re young, you believe with all of your being you can grow up to be anything you want to be. You believe there is a God looking after you, your family, and everyone you care about. You believe that Santa comes once a year if you’re on your best behavior.
You believe in miracles.
Slowly, as the years go by and a sense of 'reality' sets in, you realize a jolly fat man in very unfashionable clothes couldn’t be bringing you and all of your friends across the world presents in the same night.
You realize you don’t have the intellectual capacity to become a brain-surgeon or a veterinarian.
You understand that this fictional deity couldn’t possibly be causing you go to through what you’ve been through, & then send you to hell for all of eternity because he made you fall in love with someone of the same sex.
Miracles can’t be real once you’ve grown up.
& I never want to.