I had a teenager who was going steady with a girl from another household. He turned into an adult, but she is still a teenager. He asked her to move in, but she does not become an adult for 15 days!

Can anyone tell me how to force this girl's age to adult so the two can get married? Thanks very much!

Use merolas painting.. it is on this site and has like 312 hack options well cheats anyway. LOL
You can click on it and force a sim to age or not by adding/ subtracting days to thier birthday. I use it all the time, it is not EP required but I do not know if it is EP ready.

Or: Press control, alt and c at the same time. Type boolprop testingcheatsenabled true and then press enter. Shift-click on your sim and click 'set to birthday'. They will then age transition. Type boolprop testingcheatsenabled false in the cheat box to switch the cheat off.

You can use Sim PE to do that. Just change "remaining days in age" to 0 and commit/save. When you bring up your game again, you can click on her and select have birthday.