Train to Rennes from CDG

Use the SNCF Connect app. Give yourself about 4 hours from landing time, in case of flight delays or lines at immigration. Or just get your tickets upon arrival.

If SNCF Connect does not work for you, try trainline.com.

Keep in mind that many tickets won't be available until at least three months ahead of time. I always use SNCF, as they are the ones who actually operate the trains. Everyone else is just a reseller.

Thank you for the replies. On the SCNF app the tickets seem to be designated for “adults 30-59.” I’m 64 and wonder if I need a different ticket?

You have to click on the box that says 30-59. You will see a little arrow pointing to the right in that box. Click on that arrow and you will see the options for tickets for other age groups. You get to the selection of age groups when click on on the downward pointing arrow at the spot that you get to when you clicked the right pointing arrow.

We used the SNCF app to purchase our tickets (age 60+ from the menu) from Paris to Rennes in April.

The tickets/QR code will be in the app ready to tap at the train station.

You can only get the reduction for being 60+ if you buy a Carte Avantage Senior (which you can do at the same time as you buy your ticket online). It costs 49 €. If you're just doing a round trip CDG-Rennes, you will not save a lot - you get a 30% reduction so it depends how much the ticket costs whether it's worth it or not. (Price varies a lot depending on day/time.) For us in the summer it was worth it but prices were relatively high in July.
Actually what happened was our flight from the US was cancelled, we arrived a day later, so our carefully-planned-in-advance CDG-Rennes tickets were useless and we had to buy new tickets.