Then 7 years old is the youngest child, 13 years old is the oldest child, 13 and 9 years old are the two older children and 7 and 9 years old are the two younger children. This is not question about language but rather a question about sets to me. When you are talking about the youngest or the oldest, that should be only one of that kind.
His sister is his youngest sibling, but his younger sister. Since she is the lone female child, there is no need for the superlative form. Suppose, however, that there are more siblings, and their birth order goes like this: boy, girl, boy, boy, girl. my [young / younger / youngest] sisters - English Language & Usage ...
youngest viable fetus, 1 I find the phrase one of and the word youngest clumsy in this context, and would instead say (for example) He was among the younger siblings or He was a young sibling or He was little brother to most of his siblings. The "again" means at another time or once more as in, for example: I shall not look upon his like again — Shakespeare Today is the oldest you've ever been and the youngest you'll ever be again. is a literary quote by Anonymous that has been around for a long time, and I think that that is proof that the again is necessary. For example, you have not heard anyone quote, you'll never be younger ... "again" in "Today is the oldest you've ever been and the youngest you ...
youngest viable fetus, My youngest brother works in the back while my elder, younger brother takes orders at the counter." "The older of my brothers." Comes across as a grammatically awkward statement. It doesn't sound right when I attempt to annunciate it. I could be wrong, since I don't have a PHD and am just an ESL teacher, but it doesn't sound smooth. An only child is both the youngest and the oldest, sure. But you cannot say "the youngest of the two children", now can you? You would never ask someone which of their parents was the oldest.
It just doesn’t feel right. The older parent is older than the other, since you can’t say that the oldest parent (of the two) is *oldest than the other.
