In this clip, Ali is playing around with legendary trainer Cus D’Amato, who, even then, was an older guy. He is goofing around but D’Amato is not: he’s looking for an opening and when Ali lets his guard down, D’Amato pops him in the lip.
The man must be at least 60 in this clip, certainly no younger than 55. But he lets the champ clown and when he clowns too much, he tags him.
It’s not like it was a real fight, but you have to think Ali was furious with himself, the best fighter in the world, letting an old man get the best of him. It’s the same lesson Coach K learned in 1983: respect your opponent. Always.