"How old are they?"
Translation:Kiom ili aĝas?
No answers have been given to the question "Would 'Kiom de jarojn havas ilin?' work as well as 'Kiom ili aĝas?'", I think.
The other comments here discuss why there shouldn't be an "estas" somewhere, from what I can see.
tomaszym says both answers are correct in theory, but it isn't clear whether both could be used in practice.
My own most detailed answer is in this blog post.
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:
"Kio" and "kiom" address different things entirely. "Kio" asks "what/which thing". "Kiom" asks "how much/how many".