When you're getting a bit older, you can work harder and still compete or mix it with younger guys in one off situations, but what starts to hit is your recovery times.
No matter how fit you are and how you manage yourself, what used to be sore the next day, becomes being sore two days after, then three..and so on..and you're training ..and playing hurt and sore even though you're not injured; that's when the enjoyment starts to diminish.
When you throw in an 82 game schedule and travel, that's where you see the likes of the Sedins and their team fading away on the back nine in recent times when they've been expected to still carry the torch. They're probably a good example because they get by on their skill and IQ probably more than speed or physicality.
Reduced ice time/roles and they can still play and probably contribute going forward. Run them 20 minutes a night and they're finished, even though they're still fit and can play and perform at that level.
35-36 for me. Can still compete at a high level, there's just a lot more younger guys after your job.