Which comparative adjectival term meaning “more advanced in age” is more correct in usage? Many people still prefer to use elder and its superlative eldest, but they tend to be, well, older; the choice of that last word is becoming the alternative of choice.
One reason is that there is no word eld to serve as the basis of elder and eldest; it seems more sensible to many to progress from old to older to oldest. (There, are, however, other comparative/superlative pairs with no related basis: better and best progress from good, and worse and worst regress from bad.)
More significantly, though, is the grammatical limitation of elder: One can write, “He has an elder brother” and “He is the elder of the two” but not “He is elder than John.” (The prohibition isn’t logical, but it’s there.) In addition to “He has an older brother” and “He is the older of the two,” conversely, “He is older than John” is considered proper.
Another limitation is that elder and eldest apply only to people, but older and oldest may refer to people and inanimate objects alike. Also, although elder and eldest may refer to relative age within a family, the terms are not applied in other social contexts (besides isolated applications such as “elder statesman,” which refers to a wise and experienced but not necessarily older politician or other authoritative figure): One writes, “He is the eldest brother” but “He is the oldest child in the school.” (Keep in mind, too, that though elder and elderly imply advanced age, one does not need to be long in the tooth to be the elder of two siblings or the eldest of three or more.)
Elder is descended from the Old English word eldra, which refers to a parent or other older person. (The etymology of the name of the elder tree is unrelated.) The usage in “Respect your elders” shows its age, but the term is still employed in a religious context to refer to church leaders; an older term for church elders, presbyter, is from the Greek word presbyteros — which means “elder” — by way of Latin. (Presbyter was ultimately altered to priest.)
Elder is sometimes seen in genre fiction such as fantasy novels to impart a romantic cachet to a bygone era: “Long ago, in the Elder Days . . . .”
Elderly persists as both an adjective and a noun (“the elderly”), though some consider it demeaning and prefer older as a simpler modifier and “senior citizens” or just seniors to refer to the demographic. Interestingly, the Old English predecessor of the adjective, ealdorlic, had several superlative senses: “authentic, chief, excellent, princely.”
Older and elder, as you may have guessed when you were younger, share an origin: They both stem from a Germanic root that produce the variants ald and eald. (Adult and adolescent are related words.) The former term survives in alderman (once also ealdorman), a quaint alternative to “council member” that persists mostly in the Northeast United States and originally meant “chief, ruler,” and in the Scottish auld — as in “auld lang syne” — another variation.