This explanation is from the Notes (just replace vi with ili):
How old are you?
Kiom vi aĝas?
(Literally: How much you are-age?)
Kiom is used with age, since age is a quantity (of years).
Note: An alternative way to ask someone's age is Kiom da jaroj vi havas? (Literally "How many years do you have?")
Technically there is no decent gender neutral pronoun in Esperanto -- officially, it's gxi, but people don't usually use that. There are unofficially other pronouns people use, most notably ri and sxli, and think one or two others, but none of them are official. I think sxli is the most commonly accepted one, but don't quote me on that.
There's a table at the bottom of this page: