In formal research, which is more correct, and why:
- the group of 40-50 years old OR
- the group of 40-50 year olds
In any case the phrase in bold is to be treated as a noun only, as in:
The middle group of executives, ie. 40-50 years old is well balanced....
Without a range, the hyphenation rules I am used to would suggest "15-year olds" for instance. However "the 15-25-year olds ..." doesn't present well, does it?
There are related questions here, but none that seem to exactly address this topic. Eg.
The main difference perhaps is that I need to use ranges, which already use a hyphen.