Ive known cats that lived past 20, though Ill admit that they lost their spryness long before that. 

The oldest cat I've met was 25, and I've known several who were in their twenties.  I believe the world record is something like 38 years old.  Remember, age itself isn't a disease, it just increases the likelihood of various diseases occurring.  If a cat dodges the bullets of kidney disease, diabetes, hyperthyroidism, and arthritis, they can be quite active for a lot longer than you'd think.

Or maybe he's just part kneazel... ;-)