What is the best ticket for getting around Munich for 2 senior adults? Assuming we'll purchase at the airport? We'll be needing to get from the airport to our Airbnb (in Schwabing, 20 min away) and then daily back and forth to city center.
How long are you in Munich? And are you returning to the airport when your time there is done?
Day tickets.
MVV ticket shop: https://ticketshop.mvv-muenchen.de/index.php/standardProductSale#
MVV zones: https://www.mvv-muenchen.de/en/tickets-and-fares/tariff-structure/zones/index.html
Age does not matter.
Do you know the name of the station in Schwabing that you will be using? It looks like most Schwabing stations are on the U6 line and most of those, except stations starting with Garching- are in the central, or M zone. The first two Garching stations are on the border and can be considered either zone 1 or 2 stations. Only Garching-Forschungszentrum is only in the 2nd zone.
To get downtown from any station on the U6 line in Schwabing, except those with Garching in the name, you can use M tickets (central, M, zone only). Two of the Garching stations require M-1 tickets; only the Garching-Forschungszentrum station requires an M-2 ticket.
But none of that matters when coming from the airport (zone 5). An M-5 ticket, either single trip or Day pass (Tageskarte) will get you from the airport to any U-Bahn station. There are also some buses from the airport to bus stops on the north side of Munich, and an M-5 ticket should work for them too.
I would definitely recommend the MVG day ticket as well.
But do you mean 20 minutes from the airport to Schwabing? I can't imagine it being that fast. I'd estimate it would take more like 45 minutes if you use public transportation.
The airport is connected by suburban train (S-Bahn) to downtown Munich. However, the S-Bahn doesn't have a stop in Schwabing (by the way, Schwabing is a large Munich district), so you'll probably have to take the subway, bus, or tram (streetcar) for the last part of your journey from the airport to your accommodation - depending on where in Schwabing your accommodation is.
But Schwabing is a great district with an interesting history. For one thing, the not-so-poor Munich residents have always lived there, and it was also a hot spot for artists for a long time. And if you're interested in Art Nouveau architecture, Schwabing is the place to be. There are a lot of fantastic Art Nouveau residential buildings there.
Schwabing also has an old part (the old village of Schwabing—Schwabing is older than Munich itself) with a church dating back to 1315.
https://www.munich.travel/en/categories/discover/urban-districts/local-love-munich/local-love-schwabing
https://boredinmunich.com/art-nouveau-in-munich/
But do you mean 20 minutes from the airport to Schwabing?
I think it is more like an hour or so if you take the S-Bahn into downtown Munich and then go out to Schwabing on the U6 line, but there is the Lufthansa Express Bus. The bus is going to the Hbf, but stops at Nordfriedhof, a few steps from the U6 line station, in 25 minutes. I think it's around 14€ for the bus to Nordfriedhof. If you want to go to another station on the U6 line, you'll need a 4,10€ M ticket, and there are no group tickets for the bus so it will probably cost a little more, but you'll save half an hour.