score:8
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
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.