- It reminds me of those elderly people complaining about youngsters. - fine "youngsters" has a negative connotation to it. I presume that is intended. Otherwise - It reminds me of those elderly people complaining about young people. - It reminds me of those elderlies complaining about youngsters. - "elderlies"? not familiar with this term.

Some adjectives referring to a class of people can be used as a noun with a plural meaning, for example “the rich get richer and the poor get poorer”. “The rich” means rich people in general, and “the poor” means poor people in general. Note that we do not add a final “s” or change the noun in any way. (The only exception I can think of is "the others", which is sort of a plural pronoun.) This is a common usage with adjectives of nationality ending in -ese or -ish: “The British are coming, the British are coming”. This pattern cannot be used with adjectives ending in -an: The Americans (not the American) are coming.

I speak US English. I have never seen, heard, or used "elderly" as a noun to mean "an elderly person"; the only meaning it has for me as a noun is to refer to elder people in general. It's possible that people in earlier generations used "elderly" to mean "elderly person" and "elderlies" to mean "elderly persons", but it is not how most natives speakers in the US use the word today.

US English: It reminds me of elderly people complaining about youngsters. or It reminds me of elders complaining about youngsters. I would remove the word "those" unless you were referring to a specific group of elderly people." A slightly more casual way to say this would be: It reminds me of seniors complaining about "kids today." or a really more casual way would be: It reminds me of old guys yelling "get off my lawn."

Additonally, some are used to "The elderly" as a collective term for "elderly people".