Usually, elder as a noun is plural and refers to older people as a group, not to a specific person. “You should respect your elders.” We tend to only refer to old people as elders in very specific situations, usually when referring to them as a group to be respected (as in the paragraph above) or who have wisdom. “The elders of the tribe knew how to survive the bad times.” You could also say “the elderly.” “The elderly are good at technology.” It’s the same thing as saying “Old people are good at technology”, but “the elderly” is more polite and respectful. If you are talking about one person, you usually use elder as an adjective. “I saw an elderly woman at the store. “ Or I saw an elderly person crossing the street.” In that case, it’s more polite and respectful than saying old person.

noun Respect the elderly. (Old people in general. US only?) Respect your elders. (Respect people who are older than you) adjective My elder brother is an idiot. (would usually say older) He's the elder statesman of the club. He's been a member for 50 years. "Elder" is also a title in some religions, tribes etc. The Council of Elders will issue a decision on the controversy soon.

Elders is an old term we now use the word "elderly" to describe old people over retirement age. Elders is refined to certain situations, such as the religious leaders of a community or the wiser elder people, that can give advice and impart wisdom. And the usage is as explained by the other correctors.