Additionally,In comparisons between two persons, elder means “older” but not necessarily “old”:
My elder sister is sixteen; my younger, twelve.
Eldest is used when three or more persons are compared:
He is the eldest of four brothers.
In other contexts
elder does denote relatively advanced age but with the added component of respect for a person's achievement, as in
an elder statesman. If age alone is to be expressed, one should use
older or
elderly rather than elder:
A survey of older Americans; an elderly waiter.
Unlike
elder and its related forms, the adjectives
old,
older, and
oldest are applied to things as well as to persons.
elder 1. The American HeritageŽ Dictionary of the English Language: Fourth Edition. 2000.