"How old are they?"

Translation: Kiom ili aĝas?

[paradastein]

Why doesn't Kiom jarojn havas ili? work?

[tomaszym]

Well this is the most direct translation from English. That's it. So the correct form of what you propose "Kiom da jaroj ili havas" (or "havas ili") would work in practice, but it's not really a translation of this phrase is it?

[esposch]

Why not Ilin? Aĝas is a verb, ĉu ne?

[solidgitarius]

Ili is the subject, not the object of aĝas.

[Rae.F]

As solidgitarius said, "ili" is the subject. But beyond that, only transitive verbs take objects. Not all verbs are transitive.

[solarpistol24]

Does *kiom aĝas la hundoj?" Equal "how old are the dogs?" Or am i misunderstanding the words use?

[Rae.F]

Your sentence looks okay to me.

[WilliamHallman]

Why is it agxaj? Plural the word 'old'?

[Rae.F]

aĝas is the present-tense form of the verb aĝi, which means "to be/have age".

[Davgwynne]

Would "Kiom estas iliaj aĝoj?" work as "What are their ages?"

[Rae.F]

No, because "kiom" is "how much/how many". It would probably be "Kio estas...?"
http://www.esperanto.org/stanford/korelativoj/

[Davgwynne]

Thanks. Whilst your answer might be right, the logic behind it does not follow. If "Kiom" is right for second person then why should "Kio" be right for third person? The converse also applies. The link you gave didn't help with this question.

[Rae.F]

1st, 2nd, 3rd person etc. has nothing to do with it, although if you want to be technical, then all correlatives are 3rd person by definition.

This link explains the correlatives in greater detail:
http://literaturo.org/HARLOW-Don/Esperanto/correlatives.html

"Kio" and "kiom" address different things entirely. "Kio" asks "what/which thing". "Kiom" asks "how much/how many".

[Davgwynne]

Thanks again. The person was again a red herring. I forgot what the original sentence was as I was typing - can't see it on the screen as I type - old age and short term memory problems. Sorry.

[jbrock64]

No omit estas and just say kiom ili aĝas

[Bland00]

Why do you not include estas?

[Rae.F]

It's not necessary. "Aĝas" is the "to be old" verb.