Sunday Express, February 5, 2017
Dear oh dear. An editor should know better. He has two sons, so they are the elder and younger boys, not eldest and youngest. He could have got away with ‘eldest child’ for the first boy but not ‘youngest child’ for the second boy, because there is a daughter aged 18 who is the youngest child. I don’t think you can say ‘my two eldest children’ either; that should be ‘my two elder children’. 0/10.
Incidentally you can say ‘eldest’ or ‘oldest’ for related people, but only ‘oldest’ for unrelated people or things, such as ‘the oldest boy in the class’ or ‘the oldest rocks yet discovered’.
PS It has been brought to my attention by my son that younger people find this rule archaic and are quite happy with ‘oldest’ of two. I still don’t see any harm in doing it ‘right’.