Our two Burmese cats are eight years old and are still as playful, energetic and affectionate as when we got them as kittens - even though they are now quite a bit larger and heavier.

This appears to be a characteristic of the breed:

"Burmese maintain kitten-like interests and energy throughout their adulthood."

http://en.wikipedia.org/wiki/Burmese_%28cat%29

[Should be noted that they are also utterly gorgeous as well]