Adjectives in English never have a plural form — “elderlies” doesn’t exist. (Well, it is sometimes used as a noun, but it’s not really proper, and it sounds a bit rude to me.) It is possible to use “the elderly” to refer to elderly people in general. This is always considered plural, although the adjective doesn’t take a plural ending, we use a plural verb: “The elderly are an important source of wisdom for young people.” In all three of your sentences, you have used the word “elderly” correctly as an adjective. It sounds very natural, nice, and polite. There are a couple of unrelated grammar mistakes. This is surprising, since you’re usually so accurate: We need an apostrophe S on “grandmom” (also, most people say “grandma” or “grandmother”). We don’t say “today morning” — we say “this morning.” About your grammar teacher — it’s possible for elderly people to be open-minded. We should try to keep an open mind about the elderly ;)

Ha! You should have seen my previous profile picture, it was quite beautiful as well ;)

I clicked on your name because the bird was so beautiful. And I saw your first language. That's awesome. Just wanted to share. :-)