I agree. Most dogs of calmer breeds (probably not Border Collies or JRT's!
) mellow out as they mature, and if there is nothing else to do, will sleep. By nature, predatory animals tend to have intense periods of exercise in between long periods of laying around not doing much of anything.
Kodi was a very "busy" puppy, and while he was a TON of fun, he was also exhausting. It was like keeping up with a toddler. Now, if I'm too busy to play with him, he'll lie down by my feet or on the couch and sleep. He's more than ready to jump up and "play" though, if I say, "Want to go to school?" or "Do you want to go do Agility?" or even if I just bring out a squeaky ball for a game of fetch.
He's always up for LONG walks in good weather, but neither he OR I are big on very cold weather, and cleaning him up after a prolonged period on slushy or snowy/salty sidewalks is no fun for either of us. So in bad weather, we do lots of short training sessions indoors, and do a couple of intense play sessions. He usually chooses the times for these... typically after breakfast and then just before or after dinner. That said, he's up for a game any time anyone is willing to play with him... I've hardly ever seen him want to quit before we do, unless the weather is REALLY hot.
I really don't think Eli is slowing down because he was neutered. I think he's just growing up! I think a dog that mellows out and doesn't pester you when you CAN'T play with them is MUCH preferable to one (like a couple of the Border Collies I know!) who have no "off" button!