Kai's already acting like a senior. If he seems healthy to you and your vet, don't worry. For example, Kai's only a year old. I've never EVER seen him act like a regular dog. He's very mellow and only plays when I insist. Sometimes, he'll even decide when to stop playing.

I've noticed that he'd be a bit more playful if I feed him early (as early as possible) and if he eats all his food. He usually leaves about 25% of it but when he eats it all, he's more active throughout the day.