grammatical number - Which is it: "1½ years old" or "1½ year old"?

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

Answer

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.