I'm 74, with a thin build. Started serious riding on a mtn.bike. Only road bike now, but no problem with stamina--except if I don't fuel correctly beforehand! I do go to an exercise class geared towards seniors, twice a week. It does help, with a mixture of cardio, strength, and flexibility exercises. And I'll have a protein drink with 30g of protein, as well as a protein bar before riding, and take a sports drink along to drink while riding. Stamina can be gained but it just isn't as easy as it was a few decades ago!! I've found that rides where I didn't feel so great were ones where I skipped fueling beforehand. Unlike my car, I don't still run at full power if low on fuel.