adjective
1
BAD : The restaurant seems to be more popular with elder people.
GOOD : The restaurant seems to be more popular with older people.
BAD : The young man was unhurt but the elder one was taken to hospital in an ambulance.
GOOD : The young man was unhurt but the older one was taken to hospital in an ambulance.
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Do not use elder unless you are talking about the members of a family: 'her elder brother', 'my elder son'.
2
BAD : My sister is just two years elder than me.
GOOD : My sister is just two years older than me.
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Someone/something is older ( than someone/something else): 'Their children are older than ours.' 'Her husband is five years older.'
Elder is mainly used in front of a noun ('my elder sister') and as a pronoun ('Which of the two sisters is the elder?')