I have three almost three-year-old siblings and two from a second litter that are nine months old.
Originally, there was a marked difference between the behavior of the adults and the kittens. I always thought it was natural; kittens are supposed to be full of energy, right? And of course adult cats are laid-back!
The transformation in behavior after I started feeding raw was off the charts insane. I now have five kittens who chase each other through the house, over the furniture, up one side of the entertainment center (over six feet tall) and down the other. I know it drives my downstairs neighbors nuts (thank goodness we're on friendly terms), but it makes me smile and laugh all the time. I know my babies are doin' good.
Beyond any other point, fact, story, benefit, etc., the profoundness of that change proved to me incontrovertibly that commercial foods aren't all they claim they are, or should be. I was feeding several of the very best wet foods available (non-grain, non-fish) in rotation, and STILL, my cats were obviously not at peak health.
The other big change is the litter box, but that one benefits me more than it does them!