I can answer this question. Although my spoken English is not very good, I am familiar with the vocabulary of College English Band 4 and Band 6. I have summarized a table for you to have a rough look at first:

meaning

1, the oldest English [? ld？ St], beauty [? o？ ld？ British pronunciation is different from American pronunciation.

Specific age; (how big); Age; Old; Older; Not young; Older people;

The superlative of old;

2. Boss English [? eld？ St], beauty [? eld？ British pronunciation is the same as American pronunciation.

The oldest; (three or more people without nouns) the oldest person;

use

1, the oldest means that people live for a long time or reach a certain age, which is the highest state of being old.

This is the oldest car in the race.

This is the oldest car in the racing car.

Oxford and Cambridge are the two oldest universities in Britain.

Oxford and Cambridge are the two oldest universities in Britain.

2. Elder refers to the oldest person among three or more people.

He is the oldest of the three brothers.

He is the oldest of the three brothers.

The boss helped his father to farm and inherited his small real estate.

The eldest son helped his father farm and successfully got some lifelong real estate.