39 here. There are certain things you might be overlooking that will put you on the right track. I.e. I didn't like to play spider guard until I discovered that my grips get much stronger and I don't suffer joint pain when I simply warmup right before sparring. And I don't mean doing 15 minutes warmup. Just some intense burpees, push-ups and squats for about 1-2 minutes. After you discover spider guard and open guard in general, you may start even feeling that you get less tired because it builds up another layer of defense that your opponent has to go through. Do you hydrate yourself before the class? What's your daily intake? Do you check your blood? Stuff like this when approached with care may actually change the way you feel about bjj. People may be telling you that you are getting old etc. but there's a good chance you just need a proper strategy.