When I was ten, twenty seemed old. We had a family friend who was twenty, drank black coffee, and played the guitar. I thought he was old, but I sure wanted to learn how to play that guitar.
When I was twenty, thirty seemed old. I was standing in the checkout line with a mother and her young daughter who bumped me, and the mother said, "Say you're sorry to the nice man." Ouch.
When I was thirty, forty seemed old. My son would already be fifteen, and my daughter twelve when I turned forty.
When I was forty, fifty seemed old. Now that I'm fast approaching fifty, sixty seems old.
I'm going to keep running so that when I turn sixty, seventy will seem old.