A. Talking about people
1. older and oldest may be used of any persons:
a) comparative: older
Julia looks older than she really is.
Julia looks older than Betty.
Mike is 20 and Dave is 23. Dave is older than Mike.
b) superlative: the oldest
Mike is 20, Dave is 23, and Hugh is 27.
Who is the oldest? (Hugh is the oldest).
2. elder and eldest are restricted in use to members of the same family:
a) Mr and Mrs Paddington have two daughters. The elder daughter is called Belinda. She’s fourteen.
but: She’s older than her sister.
The younger daughter is called Judith. She’s twelve.
b) Mr and Mrs Cresswell have three sons: Arthur, Luke and Nicholas.
Arthur is the eldest and Nicholas is the youngest. Their eldest son lives in London.
c) You can also say:
My elder brother is an artist.
Who is the eldest in your family?
*Elderly people are old people. You can refer to old people in general as the elderly.
B. Talking about something (towns, buildings, etc.): older, oldest
Her car is older than mine.
Rome is older than New York.
Trier is the oldest city in Germany.
That cathedral is the oldest building in the town.