Elder and older both relate to age, but they are not used in exactly the same way.
Use elder mainly for people, especially within a family relationship. It is most common before a noun. For example, My elder brother teaches math. This use can sound slightly formal or traditional.
Use older as the general comparative form of old. It works for people and things, and it is the normal choice after a verb or in comparisons. For example, She is older than her cousin. You can also say This building is older than the library.
- Elder: usually for family members, often before a noun
- Older: general use for age, comparison, people, and things
- Older is usually the safer choice in everyday English
Both can appear before some family nouns, so my elder sister and my older sister are both possible. Still, older is more flexible. You would say He is older than me, not He is elder than me.
