Euro Rail pass with Marriage age difference.

You don't say where you will be going, but be very careful of this approach - rail passes and point to point tix often have very different rules / limitations / capacity restrictions. Often point to point tix bought in advance are much less expensive than a rail pass anyway. Check advance tix prices on national train sites (trenitalia, sncf, bahn, thalys, etc.) to get an idea of prices. Enjoy your trip!

On trains which require reserved seats (usually long distance trains), if you buy a normal ticket (what you call a "point to point"), it will include the reserved seat. Pass holders have to make the reservation (and pay for it) separately. It will be virtually impossible to ensure you get seats together, they will probably be in different coaches.

The next question is whether a Eurail Pass is worth it for your wife, they often are not. As you have given no details of your "3 fairly long trips" I cannot comment. You can usually get cheap fares if you go direct to the website of the company running the trains, and buy in advance.

Any research on rail passes should start here: www.seat61.com
It will take some work to make comparisons such as you are suggesting, and you have to do the work by pricing point-to-point tickets first. I think it is inefficient to start with the assumption that passes are automatically a good deal although it is good you realize that there are breaks for younger people.

"Fairly long Trips" can be relatively cheap (if in Italy) or not so much (in Switzerland or on the Thalys), so the exact origins and destinations make a big difference.

If you're buying a Eurail Global or Select pass covering 3 or more countries, most couples will choose the group discount for traveling together in first class. Wherever the pass can offer hop-on convenience, you would lose that benefit if one of you is always buying separate tickets.

But the good news is that you can currently (at least for the next day or two) buy the larger rail passes in second class for travelers of any age, also with the group discount. The second-class-for all offer may be extended, either sooner or later, but we have not heard a decision, yet.

Passes for one or two countries (or for Scandinavia or the European East pass) already come in second class for all ages.

If you have 3 "fairly long" trips, do you also have a few shorter trips planned? Passes for more than one country start at 4 or 5 travel days and are less likely to save money if you don't need them all. The specific trip plan would help us compare.