Spoiler for Horo's age as stated in the novel:
Best data I have is that she's 600 years old .. I've seen reports of her being 250 yrs and 800 yrs but I suspect mistranslations on those. You'd think that'd be pretty straight forward data to acquire but apparently not. I don't have copies of the novels yet to verify.
As for her *appearance*, 15yr old girl is used a lot.. but in fact, she could pass for 14-20ish, so that's an arm-waving estimate in my opinion.
edit: and a guy who actually has the novels beat me to it
