I see a lot of museums in Vienna that offer reduced senior prices with "valid pensioner's ID." I assume that is an official government document. Would they accept an American passport or is the discount only for their country's elderly citizens?
Most Viennese museums request just a valid ID/Passport/Driving License which shows the age. Same is valid in rest of Austria. Other countries might have different rules.
For example, "The Kunsthistorische Museum" in Vienna mentions
- "Pensionisten ab 65 oder mit gültigem Pensionistenausweis"
- "Pensioners over 65 or with valid pensioner card"
The EU law does not allow more expensive ticket prices for non-Austrian Europeans. Same is valid for other EU countries. No special tariffs for locals (https://www.konsument.at/markt-dienstleistung/eu-einheimischentarife). There are few exceptions such general transport tickets. I haven't seen that museums request European passports on entry and it would be a hassle to verify if a pensioner id is connected to one of the 28 EU member countries. So I guess it doesn't matter from where you are.