When speaking with my mother a couple of days ago, I read to her a message I was sending to my cousin on her behalf ending with: "… the birthday of your youngest." [implying her child]
She immediately leapt on this and said that as my cousin only had two children, the use of the superlative was not permitted and the comparative must be used instead: "… the birthday of your younger child."
She was adamant that this was a solid grammatical rule that she was taught throughout her education. As a younger Briton, I have relatively little formal grammatical education to older people who were better taught in this regard, so I usually defer to her on grammatical rule knowledge.
Is she right? Are superlatives not permitted when the domain of the object is only two?
To me it seems bizarre. The minimum or maximum of any set does not only exist when there are different minima and maxima, or indeed something which is neither.
Best Answer
It certainly used to be a rule, but there are grounds for ignoring it. This would be strongest where you were uncertain of the number of children (not the case here). But you can certainly argue that in logic, there is an oldest of 1, 2, or more children. So, your mother is almost certainly correct about what she was taught. Whether that stricture still has to be followed is considerably more debatable.