Learn English – Pluralization rule for “five-year-old children”, “20 pound note”, “10 mile run”

A 16-year-old girl:
1) "16" and "year" are linked by the hyphen to create a single term ("16-year") that modifies "old."
2) "16-year" and "old" are linked by the hyphen to create a single term ("16-year-old") that modifies "girl."

She is 16 years old:
1) "old" is a predicate adjective for "girl." (as in "She is old.") As a single-word adjective for "she," it is not joined with a hyphen to the previous adjective, "years."
2) "years" modifies "old," and "16" modifies "year. "16" and "years" are not acting as a single adjective for "old," so they, too, do not need a hyphen.

The easy way:
If the adjective string serves as a single modifier before the noun, it needs a hyphen (or hyphens, as in this case). If it is after the noun, it doesn't need a hyphen (or hyphens).

This is the same as "3-day weekend" and "The weekend has 3 days." In the first case, "3" and "day" are linked to create a single term that modifies "weekend." In the second case, "days" is the object of "has," and "3" is a modifier for "days."

If you are using it as a compound adjective or noun, as in your example sentence, it should be "40-50-year-olds".

If you are using it as a separate qualifier, as in BillFranke's suggested alternate wording, than it would be "those 40-50 years old".

Confusing, perhaps, but the general rule is that when any sort of counted "thing" is used as an adjective, the object of the count is singular. "40-year-old man", "3-mile run", etc. Making it a range instead of a single number doesn't change that.

But when a number and an object of that number are used "on their own", i.e. not as a compound word, the normal rules of pluralization apply: "those 1 year old", "those 2 years old", etc.