1. We use elder and eldest as nouns:
ex : Jack is the eldest of four brothers.
2. Older and oldest are used in similar ways:
ex :My older sister is coming to stay with us at the weekend.
3. Older and oldest can be used to refer to the age of things more generally:
ex : The town hall is by far the oldest building in the whole region.
4. We can use older and oldest after a linking verb (as predicative adjectives):
ex : I think her grandfather must be older than her grandmother.