I have about twenty individual tickets for the Paris metro system from previous trips . Are these still valid for a trip this fall? I would prefer to utilize these if valid.
Tickets are good for a while -- either 5 or 10 years, I forget which. If they are unused but end up demagnetized, the ticket booth person in a staffed station will take them and give you a replacement.
Tickets t+ are good until used.
I have a stash at home from stray tickets I find tucked away in nooks and crannies. They always work when I take them back on the next trip
Sometimes, the magnetic strip on the tickets stop working. Do not throw the ticket away: if that happens, go to a booth, hand the faulty ticket, say it does not work ("ça ne marche pas"), the person behind the window will test the ticket and hand you a new one.
Tickets are good forever or until they get rid of the ticket system which is in the works. I have used very old tickets many times. As noted, if unused but demagnetized the clerks at the info windows will replace them.
The Metro tickets, valid on buses too, have expiration date. I always have Metro tickets left over from past trips so that I'll have them ready for the next trip to Paris. I tend to take more buses now than the Metro.
I used some ancient purple tickets last summer - they worked fine. They were many years old.
Don't worry about having to communicate in French to the person at the ticket booth if your ticket doesn't work. I usually start with Bonjour...then go with "Le Billet est uhh....and then make a thumbs down signal and a frownie face." If I'm flustered, I've also resorted to just sign language after the initial Bonjour. The ticketsellers have been extremely nice.
BTW, not sure if you know but magnetic clasps on a purse or wallet can zap them. I usually base my purse/wallet/tote/backpack purchases on NOT having a magnet!
as Pam notes you can get far with Bon jour, sil vous plait and merci and pantomime -- back in the day when I had even less French than I knew now I have successfully gotten directions to a butcher shop, obtained towels for the room (we had towels for 2 in a triple) and communicated other needs with almost no French but lots of gestures and humility. Little old ladies with no English have escorted us to the beginning of a path we needed to take. If you are amiable and polite, you will have no trouble dealing with minor ticket issues. (just never be caught without a valid ticket in the system though -- no quarter is given.