We will be traveling to Provence in May and will be using trains and buses to get around. The train tickets for seniors seem quite cheap, but do seniors need a card of some sort in order to get the cheap senior tickets?
SNCF has a Senior+ rail card, but I believe it has been mentioned here that some level of senior discount is sometimes available without the card. I suspect this is one of those situations in which you need to do the math both ways and compare the difference to the cost of the card. If you're sticking to Provence, my bet is that the card won't pay off, but I am just guessing. €60 is rather a lot of money, and you'll probably find yourself using buses quite a bit in Provence.
The card is valid for a year, so if you expect to return to France for another trip before the card expires, that could certainly tip the balance.
Go to the SNCF website to check fares with and without the Senior+ Card.
On TER trains, you only need ID (passport) to prove your age if a conductor checks your ticket and questions your age.
I had senior rate tickets on the TGV from Paris to Nice and didn't need a 'senior card', just my ID showing I was over 60 or 62 or whatever the age limit was. Purchased ahead of time online at the senior rate.
The senior card gets you 50% off, whereas just being 60 and over gets you 25% off without the card. These fares are available outside rush hours.
According to the SNCF website, the Senior+ railcard gets you 25% off any journey and up to 40% off 1st class travel. It costs €60 so it pays to look at advance purchase senior rate fares on your planned itinerary to see if it's worth it to you. It gives you the opportunity to get the 25% even during peak travel times rather than just at non-peak travel times as Bets notes and it also gives you a couple of other benefits. I looked into it before my trip to France because I was going to take several train trips but it wasn't worth it for me.