Hmm. I don't like how "I have an eldest child" is used to imply "therefore my second-oldest child can't have the same age [when rounded down to a year number]"

Hmm. I don't like how "I have an eldest child" is used to imply "therefore my second-oldest child can't have the same age [when rounded down to a year number]". As the article says: "although one could be a few minutes or around 9 to 12 months older". I could imagine parents objecting to the claim that one twin is older than the other (or even saying they didn't keep track of which was born first); but a child having been born 10 months before another is something I'd expect parents to remember, and I don't think it would be weird in that case to call the older one the "eldest".