I'm planning to buy museum passes for myself and my mother to save time and money. I was not planning on getting my 17 year old a pass because she should be free or reduced cost, but if I don't get her one, will I have to wait in the long ticket line?
No she just walks in with you. She needs ID to prove she isn't 18; she may not be asked but at 17 odds are fair she will be.
The good thing is her passport will have her birthdate.
Enjoy your trip.
If she has other photo ID with her birth date, that should work to - driver's licence or maybe student ID, so you won't have to keep trotting out the passport. I've always been able to use my driver's license for proof of age, though I'm at the other end of the discount scale :-)
There are a couple of places where you do need to wait in line to get a "free ticket" for your 17 year old. Two that I know of are the Musée de l'Armée and the Arc de Triomphe.