Anybody who thinks Garnett looks older than Duncan on the court has definitely not watched many Nets/Spurs games. Duncan makes KG look fresh out of high school.
The only difference is Duncan never relied on much athleticism (not anywhere near KG). He's also on a slow, methodical team, not trying to keep up with a group playing small ball.
The whole "Kevin Garnett is cooked" thing I keep seeing on this site is a myth. He sucks compared to 1-2 years ago when he was a top 5 center, but so what? That hasn't stopped him from leading the league in defensive rebounding percentage.
He's still better than half the centers in the league. The things he does on the court for a contender still dwarfs the contributions of a Kendrick Perkins. Put KG on a team like the Heat/OKC/Spurs and nobody would be saying he has to retire.