FWIW, I hit the best shape of my life at age 40, after having a kid. Lowest weight as an adult, too.
It is harder after age 20, physiologically. But in other ways, it's much easier. Fewer weekend evenings out with friends, for one thing!
I suppose you could plug your ears and sing "la la la la la", but that wouldn't be very adult...
//b. strong,
Kim