How to punctuate "eight- or nine-year-old" [duplicate]

I am writing about some leadership stuff, and I am trying to say that I am leading the 14-15 year old boys. How do I correctly use year-olds as in, "I was the adviser to the 14-15 year-olds."

Thanks in advance

Use a suspended hyphen:

I was an advisor to the 14- and 15-year-olds.

I would use a suspended en-dash:

I was an advisor to the 14– and 15–year-olds.

The choice of open (year old), closed (yearold) or hyphenated (year-old) compounds is often a difficult one. The first guide is to see what others do and whether the compound exists in a dictionary. This is not foolproof, not least because some exist in all three forms (eggbeater, egg-beater and egg beater are all commonly found).

Failing that, avoid the closed form (yearold) if you don't see it often. Closed compounds are relatively rare in English (though becoming more common), so it is safer only to use them if they are often used by others or have a separate dictionary definition not marked as rare, obscure or obsolete. This is the one possibility where you can end up picking something most people would say is wrong rather than merely less than ideal; people will object that there's no such word as yearold. However, that very same fact is a reason to favour the closed if it is found and if it is found with the meaning you intend, as it will be the more precise. Becoming a new word, as closed compounds do, gives them a further strength.

After that, unless the hyphenated form is particularly common, I would lean to the open unless it was ambiguous in context. In this case the hyphenated form is just about common enough to make me lean toward that over the open "year old", but there's an element of taste and personal style at this point, so I would not say the open "year old" was incorrect.

If used as an adjective then there's yet more reason to favour the hypenated:

I was an advisor to the 14– and 15–year-old boys.

Open ("…year old boys") is also valid and grammatically correct, but there's just that extra bit of visual assistance to the reader in stressing that "year old" is a single modifier, so their eye splits it [year old][boys] rather than [year][old][boys] or [year][old boys] on first glance. There isn't a risk of confusion or ambiguity here (there might be in some analogous examples), it just prevents the reader having to do a double-take and correcting themselves.

For this reason compounds that work well open as nouns may well be better hyphenated as modifiers.

When combining more than one compound, the en-dash (–) is applied in joining to what is already an open or hyphenated compound. Hence we first have the compound "year old" or "year-old" and then the en-dash further compounds this "15–year old" or "15–year-old".

In this case choosing this over the more open "15 year old" or "15 year-old" is particularly beneficial as it lets us combine it with the previous "14" to create two compounds; the suspended dash of "14–" tells us that what is done later to "15" is also done to "14" and lets the reader expect that.

Both "14–15 year-olds" and "14–15 year olds" are valid, but allow a temporary misreading of "14–15" as a number of children rather than their ages. (Note incidentally that I use the en-dash "14—15" here rather than the hyphen "14-15" for a different reading, that dashes are preferred over hyphens for ranges. This style is common, but not universal). If forced between one of the two I would use open "year olds" for almost the opposite reason as in the adjectival use above; I want to avoid visual confusion that breaks the phrase into "14–15" as one unit, and "year-olds" as another and leads to a brief mental re-adjustment. Again though, either is grammatically okay.