The usage of elder,eldest, older, oldest

Elder and eldest are more restricted terms than older and oldest. They can only be applied to people, whereas older and oldest can apply also to buildings, mountains, newspapers, customs, and so on. They can be used in only a limited range of grammatical constructions.

Both elder/eldest and older/oldest can be used either as adjectives-

Sam is the elder/older brother; Molly is my eldest/oldest child.

As nouns, Sam is the elder/older of the two boys; The eldest/oldest of my two children is Molly.

However, elder and eldest can't be used as adjectives detached from their nouns. If detached adjectives are used, then older and oldest have to be used.

• Is Sam older than Tim? No, Tim is older.
• The child who is oldest is Molly.

Elder and eldest can't be used in these sentences. If they are detached from the nouns then they do not function as adjectives and become like nouns themselves, preceded by the, my, and so on.

• Tim is the elder; The eldest of the two is Molly.

Older and oldest are equally appropriate here.

Where an or my, for instance, is used immediately in front, rather than the, then elder and eldest are preferable.

• He has to shoulder family responsibilities-after all he is an elder son; Molly is my eldest.

Older and oldest would sound awkward here though they are not altogether impossible. Yet another difference is elder can never follow than, whereas older can:

• Tim is elder than Sam-----incorrect.

There are a few common cases in which elder is used outside the context of family relationships.

As an adjective, elder can mean 'senior' when referring to two specified people:

• Mr. Kapoor is the elder partner, I believe-not Mr.Sehgal. 

Here, elder probably means 'older', though it could also mean 'senior', I.e in the sense of serving long.

An elder statesman is an elderly person or retired politician, who acts as an advisory.

As a noun, elder can be used, usually in the plural, to refer to anyone who is older than the person addressed or referred to:

• That young lad has no regards for his elders.
• He is my elder by a good five years.

Village elders, usually limited to men, are the elderly and influential members of a small community.

A church elder is a parishioner, usually unpaid and a layman, who shoulders various responsibilities in the church's affairs and proceedings.