Before learning about Trump's age, I honestly thought he was around 55-65, I was surprised he is about the same age of my grandfather. He does not look old, in my opinion.
Besides, I think that people say that Biden is old and similar things because he seems to have cognitive problems, which is unfortunate, specially because it has become a political narrative.
It is not about keeping an standard, but attacking the opposite party.
People who dye their hair often seem younger and good hair is signaling vitality, and vitality is signaling competence etc. And trump has definitely put lot on effort on looks and prestige, and believes that if he delivers himself as believing he is smart, some people would believe him if he was persistent and aggressive enough about it.
Other ways to do this being arrogant enough and not only demand attention in conversations but take it. So much it's pretty much all me-me look at me and my brilliance, because he understands that screen time is everything and some people who totally believe what he says because no suit has acted like that ever in tv. This is one of the reasons why he seems so insecure of someone taking the spotlight, he is losing if they don't watch.
Now that people have learned that this is what he is aiming at, there isn't as much spotlight for him and he can't dominate the media by trolling. Meaning that he will probably won't make very far this time.