Translation:The elderly people drink coffee and tea.
In the parts of the US where I have lived, the word "elderly" has the negative connotation of being both old and debilitated in some way. People I know in their mid-80s would be offended to be called elderly. So would say : the older people" rather than the "elderly"people. It is not a literal translation but it is more polite.
Nouns that end in the suffix "-er" (which is common among words for professions) don't change in the plural, but "Senior" doesn't end in this suffix. (The are even some nouns that end in "er", but not in the suffix "-er", that change in the plural, but I can't think of any that are professions, unless you want to count "Mutter" and "Vater" as professions.)