Hi from Wisconsin,
We were in Czechia last fall and made a loop on the trains: Praha-Brno-Olomouc-Praha. It was under $40 per person. Praha to Brno might have been $7. For those prices I didn't work too hard finding out how to get a discount. It seemed to me I found two answers, you must be an EU resident, and the other one that suggested your social security card or other government ID should work. But keep in mind, not all train personnel speak or read English.
Czechia is not a wealthy nation and so many things are so inexpensive, a hotel apartment for $52 night for instance. I don't mind paying the 'full fare' of $7. I book early, so you know I don't want to over pay, but I do want to support their mass transit.
As with most all train tickets, book early. And an easy way to book Cz tickets is cd.cz. Or I think loco2.com would work. cd.cz is the Czech national website for their train system. And loco2 is a British website that deals in train tickets at no mark-up.
I would put in 'over 65' and see what happens. But book early. and become familiar with the cd.cz web site. You can book a ticket and for no extra fee a specific wagon and seat. That way, you know way in advance which wagon to look for and then that a seat will be available in that car.
You might also familiarize yourself with the Czech word for platform, train, wagon, seat, hello (dobre den sounds like dobray den, on a keyboard it is C-E-C) and the please/thank you word (prosime: sounds like pro seem). I use 3x5 cards for important words. One card for train words another for plane words. Blank cards so I can write down the name for a taxi ride destination. I guess a modern person would put their faith into a battery operated phone's memory.