Well, that's because the season of a hockey player spans over 6 or 7 months. Include the team training camps/exhibition games, and you have the players being active from September to possibly June. To that, add all the traveling involved.
The training camp in fighting is the equivalent of the off-season training of team sports, and then you jump right into Game 7 of the Stanley Cup. Cormier fights maybe twice per year? Look at Roy Nelson, this guy made the UFC and had a successful run. He was never one of the best ones, but overall, he's had about 20 UFC fights, to me that's a successful career. There is no effin way someone with his physical condition could even come close to competing in any league as grueling as the NHL.
In no way shape or form is being a fighter in the UFC similar to an athlete in the NHL.
Didn't Bernard Hopkins unify titles at 49 in boxing? Randy Couture was winning titles in his 40s too.
Fighting is very very different. You can train all year and fight once. Heck, it happened to Cormier when his only fight over the span of about 15 months was to Anderson Silva. You can't possibly compare that to hockey players who need to travel to multiple cities every week to play 2-4 games.
Your reflexes can take a step back, but then, will your opponent be able to even use that to his advantage? Not necessarily. If I try to box Hopkins, he'd crush me in one second even at 53. Of course, the discrepency between actual fighters is nowhere near this level, but point is, if your opposition isn't that elite to begin with, your age won't matter all that much. That's why I give him way more credit for his Stipe win, who I feel is very good, but then he's 36 so the age difference isn't important.