BJJ will usually hurt you more than Muay Thai. At your age, BJJ will hurt your back first, esp. lower back (not including submissions & injuries b/c you didn't tap early). Muay Thai, then it'll be more cardio and pain to injury on the ankles, knees, etc. Don't be shy about wearing a brace for the back, ankles & kness right away.... later, shoulders, elbows....
It also depends on the gym and how hard they go. Most old guys don't last long when they start out if they try to keep up with the rest of the class. I bet many people here can't do the full BJJ warmup exercises w/o taking a rest. Nothing wrong with that, just take it slow.