In The Netherlands, there are no age related discounts on trains*. On buses, trams and metro services one can get a discount, but a personal OV-chipkaart is required and the discount will then be applied automatically.
The personal OV-chipkaart can be ordered online at
OV-chipkaart - Apply for personal OV-chipkaart. I don't know however if it will accept a UK adress, the website does state that PayPal is a payment option for residents of Belgium, Germany or Luxembourg but I couldn't think of any reason to limit PayPal to those three countries.
*) With regards to trains the regional operators are an exception. On the routes mentioned below the train tariffs are integrated in the bus/tram/metro tariffs and work the same way with a base fare and a fixed fare for each kilometre. This means that the automatic age discount for passengers of age <12 of 65+ with a personal OV-chipkaart is applied on these routes:
- Arriva Gelderland: Apeldoorn - Zutphen, Zutphen - Winterswijk, Winterswijk - Arnhem Centraal and Arnhem Centraal - Tiel
- Breng: Arnhem Centraal - Doetinchem
- Keolis: Kampen - Zwolle, Zwolle - Enschede and Oldenzaal - Zutphen
- Arriva Vechtdallijnen: Zwolle - Emmen and Almelo - Hardenberg
- Qbuzz: Dordrecht - Geldermalsen
- Arriva Limburg: Maastricht Randwyck - Heerlen, Maastricht - Heerlen (- Aachen), Nijmegen - Roermond, Maastricht Randwyck - Roermond, Kerkrade Centrum - Sittard
- Domestic travel with the international trains Enschede - Dordtmund/Münster (DB Regio, check in with Keolis) and Arnhem Centraal - Düsseldorf Hbf (Abellio, check in with Arriva Gelderland/Breng)