I'm talking about in the show not in real life. I don't think they really mentioned it. I've only seen one reference in which Willis calls Kimberly "my little sister", but I don't know whether he's talking about his height or in terms of age and that was in the senior queen beauty contest ep. Also were they in the same grade in school? And how apart in age is Arnold from them?

i believe kimberly was a year older than willis. i remember in the first episode i think it is mentioned that willis was 12 and kimberly was 13. as for arnold, he was 8 when it started.

Actually i think Willis might be older than Kimberly because one the first ep
Kimberly said welcome little brother welcome big brother.
Also on the second ep called the Social Worker willis said im 13 trying to get out of here
They always study together!

I think willis is!!

Kimberly is older!!Because in pictures when all of them are younger kimberly looks the same!!