At 71 I plan on using my motorhome for another 12+ years... bought it 12 years ago. This will get me to the age of my role model... A couple from SC who had just bought a new 45 foot Beaver motorhome. They had driven it from SC to OR in 3.5 days for a Beaver rally. She was limping because of a broken leg snow skiing the previous winter.
I do not think it's your age but your activity level and how much you focus on driving. I love driving my MH... It handles well and you're up high enough to see over most vehicles. Back to back 12+ hour days are not a problem.... when they become a problem I'll drop down to 8 hour days.
Yesterday was a fairly windy day but I went flying by myself... partly to look at a subdivision that I'm helping a friend clean up. Granted there's a lot of satisfaction showing those young whippersnappers that they can't even come close to getting the amount of work done in a day that a old fart can get done.
My point is do not base it on age. We are not all blessed with still being able to do what we did at 40... OK, the aches and pains are greater. I pity my kids when they have to take the airplane keys away. Hopefully I will have the judgment to turn the keys in and not go flying alone.
OP, it concerns me that you have brought the subject up as it's the last thing on my mind. Hopefully at 75 I will be able to say the same thing.