ONLY in HUngary this is! In most countries you can buy a pass-reduction card for various prices that then gives discount.
Well, you are right that in many countries you need some type of "senior" card in addition, but there are still quite a few countries where discount is given without any, first few coming in mind are Slovakia, Lithuania, Estonia, Ukraine, probably some other too
I have made an overview-will try to put online shortly.
Yep, it would be great if you can share it. We don't deal a lot with Senior discounts, so it would be a good thing to have some kind of overview