Er, actually ipkus1 is right, she is over 18.
I've only seen one and half episodes of Kim Possible, but according to wikipedia, the final two episodes of the final fourth series were Graduation Part 1 and Graduation Part 2.
http://en.wikipedia.org/wiki/List_of_Ki ... on_4:_2007 If she's graduated from school it's pretty safe to say she's over 18 now. (She wasn't some genius prodigy who graduated early.)
When Kim Possible STARTED she was a teenager, but she's an adult now. If you think about it, she has been since 2007. She's 20 now!
Just because the cartoon finished, doesn't mean BAM she vanishes out of existance. Kim Possible wasn't a cartoon like The Simpsons where everyone is perpetually the same age and nothing changes. The couple of episodes I saw would indicate that it did have some continuity, they mentioned some things that must have happened in previous episodes.
A bit like Buffy the Vampire Slayer - when the series started she was a teenager in school, but as time went on she finished school, went to university, and then finished university.
Also, according to
http://en.wikipedia.org/wiki/Kim_Possible_(character)#Alternative_versions
"An older version of Kim was introduced in an interactive featurette that was included in the DVD of A Sitch in Time. Little was revealed about Kim's older self, except that she was still fighting crime in her 40s, was a member of the PTA, and that she wears a patch over her left eye, ala Nick Fury."So if ipkus1 says "Kim Possible is 18", well, we take it as a given she's 18.
There's plenty of fan-artworks and fan-fiction of anime and manga characters where they are supposed to be a few years older than their original appearence. I don't see why Western cartoons should be any different. Kim Possible can be as old as ipkus1 wants her to be. And given even the official "canon" state of the character is that she has graduated school, that doesn't require much imagination.