I have not found a site concentrating them into a list, but that is because the reductions are so common.
Everywhere I have traveled in Europe people over 65 get reductions on museums and often on attractions. The age limit can be as low as 55 in some countries.
Most of the time you have to proof your age, with a passport or with a dedicated '65+' pass. Mostly also tourists get the discounts. So if you travel in Europe and are over 55/60/65 (depending on the country) (or under 26 for youth discounts) bring proof of age, your passport is the best bet but an official ID card might be accepted as might be a photo driving license.
Within the Netherlands people over 65 get discounts when traveling by bus. But only when they have a personal OV chip pass, which they can only get when proving their age. I think an address in the Netherlands is needed to get the pass to you, but you do not need to be a citizen to get it.
On the tourist page of the OV Chip site you find how to apply
Staying in the Netherlands for a longer period?
If you will be staying in the Netherlands for a longer period, you can also apply for a personal OV-chipkaart. You can then add a pass to this and you will receive an automatic age discount. If you live in Belgium, Germany or Luxembourg, you can apply for a personal OV-chipkaart online. This card will then be sent to your home address. If you live in another country, you can purchase an application package for €10.50, at an RET, Arriva or Connexxion counter. You can use this package to apply for a personal OV-chipkaart. The OV-chipkaart can then be sent to an address in the Netherlands, such as your hotel.
For discounts on the train in the Netherlands you need to buy a yearly discount pass and on top of that you can buy 7 day passes for a reduced price, one every two months and one extra, not useful for tourists.
If you travel with a Dutch person who has such a reduction pass, age is not important in that case, and you also travel on a OV chip pass, you can get the same reductions they do. You need to 'load' the reduction on your pass every time before checking in, but as it saves money, most people do not mind it.
In England all senior citizens can get a bus pass for free travel, in all the country. In some areas the trains are also included for local people, but most people do not get train reductions.
Scotland, Wales and Northern Ireland also have bus passes, see the sites for the details.
I have not checked out the rest of Europe but I guess you can find discounts for rail or bus, only for locals or for all, in most of Europe.
In many cases there are reductions for those 65 and over on trains but the reductions you get on early booked tickets for dedicated trains are bigger and do not give extra reductions for older people.
When you are buying on the day, or full price tickets for whatever other reason, do try to get a reduction, but always check with the staff so you are sure you qualify before you buy the ticket.