You gotta use a few cheats... I can't remember what they are but... something like this: you can download a mirror that let's you control visitors at
http://www.simchaotics.com/
Put the mirror in your house.
When the girl comes around your house, control her. Then, use the code
boolProp testingCheatsEnabled true
and when you've done that, press down Swift and click the girl. I'm not sure, but there maybe a choice that let's the sim skip to her/his next birthday. It works on the sims that live in that house, but I'm not sure... :/
Hope this helps!