adjective
BAD : John, her elderly son, is still at university.
GOOD : John, her elder son, is still at university.
◆◆◆
elderly = old: 'an elderly man with white hair and a stick'
elder = (of brothers, daughters etc) older: 'our elder daughter'
adjective
BAD : John, her elderly son, is still at university.
GOOD : John, her elder son, is still at university.
◆◆◆
elderly = old: 'an elderly man with white hair and a stick'
elder = (of brothers, daughters etc) older: 'our elder daughter'
Longman Common Errors English vocabulary. Английский словарь распространенных ошибок Longman. 2012