What is a masterclass in AmE?

I am a non-native speaker of English. I was under the impression that "masterclass" means "a class or activity of top quality"

masterclass

• a class taught by someone who has an expert knowledge or skill in a particular area, especially in music
• a situation in which someone does something extremely well or badly, showing how it should or should not be done

source: Cambridge Dictionnary

In the course of a discussion with an American friend, he told me that masterclass means "a class for older (senior) people".

Is this an accepted (or usual) meaning in American English (or some specific dialect)? Or in English in general?

As an American, I can say that the Cambridge Dictionary definition you quoted is exactly right for American English as well, but your summary of it is a little off. It can either mean "a how-to class by an expert" (which could be good or bad), or "a performance so good it exemplifies all the things you would teach in a [definition 1] masterclass". The second version of definition 2, where the performance is so bad it is a class in how not to do it, is not something I would have included as part of the definition, but I could see someone sarcastically using it that way.

It is plausible that a service offering master classes might be popular among older people, giving the mistaken impression that "being for older people" is part of the meaning of the word. But it is not.