We should have an age limit for presidents. At nearly 80, Trump is performing at or above average for his age. The variability at that stage of life is enormous. Some octogenarians are frail, reliant on walkers, and struggle to form coherent sentences. Trump is clearly operating well above that level. He's also demonstrably sharper than Biden was in his final years. That said, some cognitive and physical decline is simply inevitable at that age.
You are correct it appears that Trump is less frail and seemingly mentally sharper than some 80-year-olds. Everyone has good days and not so good ones. Both Biden and Trump sometimes stumble.
Often the media calls Trump out for dosing off during meetings. If he is up most of the night posting on Truth Social, he likely suffers from insomnia unless he is a late sleeper like me. Folks with insomnia are known to dose off during the day. If I don't get enough sleep, this sometimes happens to me, especially if I am watching a show that is not particularly interesting on television. Today, I read an article suggesting we might need less than eight hours sleep, but for most of us not a lot less. Less than six hours of sleep a night may be problematic.
I don’t know about Trump, but I cannot sleep on a plane. Thirteen hours plus flights overseas are exhausting for me. I usually need a day of recovery.
It is worth mentioning that in our 80s, the body’s main balance systems naturally slow down. Our eyes are slower to adjust to changes in light and depth. The nerves in our legs and feet send transmit messages to the brain less quickly. A gradual loss of balance over years is normal with age and fear of falling increases.
All in all, I think old age sucks. And it sucks worse for some of us than others. Being constantly in the spotlight and under scrutiny would make these signs of aging more obvious and thus feel worse.