I don't think it's the age that could be an issue, but comfort level in general. For example, I would not feel comfortable sending one of my next door neighbours through YYZ T1 International due to accessibility things I have seen myself and how frail they are (in their case, I would ask for a wheel chair but that's another story). Sure there will likely be people who can assist along the way (and I'm pretty sure there would be FT users who could assist as well depending on lead time). I would be using a combination of the things above...although the difficulty in obtaining some of those things might be an issue. Familiarity with technology can be overcome as well. In addition to the above, what I would do.... (keep in mind, I didn't see if the OP is at the departure or destination)

Beforehand
- See if you can get photos of a gate, some signage and other points of reference at all the airports... pretty sure some fellow FTers can assist if you ask nicely... some of them might even be taking the same route that the photos are exactly what they need.
- As someone mentioned, print out a few phrases in local, english and french that could be helpful. Asking for help at the home airport shouldn't be an issue, but CDG could be a stumbling block.
- Checking in and choosing seats in advance is always handy. Less for them to worry about and keeping them together doesn't hurt.
- Let your parents know what to expect at a high level (boarding, meals, etc..... maybe show them a YT video or two about how to use the IFE)
- Go through with your parents what happens if there is a misconnect.
- Go over what to expect once they hit Montreal (eg, immigration and luggage)

At the departure airport
- If you're able to help them get checked in at the airport (pre-security), that wouldn't hurt. That could be you (if you're at the departure airport) or a relative familiar with flying. If possible, arrive earlier than suggested and maybe have a light meal with them (don't laugh, it helps) there.
- Review process timings with your parents... I know many people get confused with boarding time vs. departure time and the need to get organized at the gate.

At CDG (never been there but heard it's intimidating)
- (if they're still stressed) While on board the first flight and if the transfer timing is long enough, have them ask some of the cabin crew to help them get to the transfer point that leads them to the departure airside. That's often where a lot of stress comes up.
- If they have a map of how to get to the transfer points, that would help as well.

At Montreal
- Map and some key phrases is helpful.
- Assuming someone at the destination is familiar enough that they can be reached if assistance is required.

Other key things...
- At the departure airport, disable data, but make sure their phone is working if they need to reach out. It might wreck havoc with the phone bill, but that they have a lifeline is worth it.
- In the age of bluetooth trackers, have them carry an airtag or Samsung Smarttag. That way you can hopefully track them through their journey. May be a mixed blessing but at least you know.

Hopefully this is helpful. I've had a few friends (elderly and not) who haven't flown at all or in a really long time (as in 2+ decades) and them having someone they can guide and call for help makes it less stressful for them.