...also, don't be surprised if she ages up between you issuing the invitation and her actually appearing on your doorstep. For what it's worth, I've had better luck with babysitters than newspaper deliverers. Fingers crossed for you!
If it doesn't work, try hiring maids and repair technicians and having pizza delivered, in hopes that some other NPC will suffice? Also read the newspaper to keep an eye out for new people moving in.
(Rica, there's a quick way to check what age a sim actually is - start the "Throw A Party" process, and it'll give the actual age category in the list of potential guests. Then cancel the party unthrown, unless you actually wanted to have a party... won't tell you how many days they have left, but it'll at least let you make sure you've got a YA instead of an adult.)