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1½ is not yet 2 or more, so which do we properly say: "1½ years old" or "1½ year old"?

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  • Yet another numeric adjectival question. Yet another quantity-vs-quality question. Feb 1 '15 at 6:49
  • You can also say the child is 18 months old. Alternatively, "He's one and a half" would be understood perfectly (presumably one would already know the child's gender). I think the full written form is preferable, but there's no one to stop you from writing the number in digits: "He's 1½ years old" is also fine.
    – Mari-Lou A
    Feb 1 '15 at 7:37
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If the entry is part of a classification:

That kid is a one-and-a-half-year-old.

If the entry is describing the age of the person:

That kid is one and a half years old.

Both of these work, and work similarly for whole numbers:

That man is a 50-year-old [person].

That man is 50 years old.

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    It would come much more naturally to a native speaker to say not "That man is a 50-year-old" [note also the hyphenation here] but "That is a 50-year-old man"; similarly, not "That kid is a one-and-a-half-year-old today" [a construction I have never heard anyone use when referring to half years as part of someone's age], but "That is a one-and-a-half-year-old kid" (omitting the 'today'), or more likely, "That kid is one and a half years old". You would only include 'today' if today happened to be the child's half birthday.
    – Erik Kowal
    Feb 1 '15 at 4:04

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