senior discounts

I don't think so but you can always ask. So far the only country that WE have found that gives senior discount regardless of nationality is Spain.

Are we talking about countries giving discounts for seniors, or giving the same discount for senior to non-residents seniors?

I don't know about Norway or Sweden, but in Germany, although there are not a lot of senior discounts, when there has been one I've never been denied because I'm not a resident.

Norway - yes, at least in some/many cases. E.g., an Oslo pass, which covers a number of attractions and public transport in "Zone 1", offers a senior discount ("senior": ≥ 67 y.o.). Ditto for the state railway system, NSB.
https://www.visitoslo.com/en/activities-and-attractions/oslo-pass/prices/

Olslo Norway transit passes also have Senior (67+) pricing:
https://ruter.no/en/buying-tickets/tickets-and-fares/24-hour-tickets/

The senior discount applies to:

Everyone above the age of 67

The blind

Persons with a Norwegian disability pension

Spouses/registered partners of the person entitled to a senior discount

Note that you must prove that you are entitled to a discount if asked
by staff. If you don't have valid proof, you may be fined if there is
a ticket inspection.

With proof of age (your passport): NSB rail tickets between cities are 50% off for age 67+. Swedish Rail offers Retired/Senior Citizen train tickets from age 65, but just booking early also gets you a great discount. Sweden operates the direct Oslo-Stockholm or Oslo-Goteborg trains.

Rick Steves' Scandinavia (p. 695) says, "While discounts are generally not listed in this book, note that liberal Scandinavia is Europe’s most generous corner when it comes to youths (under 18), students (with International Student Identity Cards, www.isic.org), seniors, and families."

We were so pleased to get discounts unlike in other European countries where you had to be a EU citizen. The transit pass in Stockholm and from Arlanda Airport were a significant savings.