One of the public site (PID) said that senior citizens can ride free providing that they have an EU ID, or to an 'pass' issued by PID. Here is the comments from the website -
'SENIOR FROM 65 YEARS
Prague – travel for free
Seniors from 65 years of age travel in the area of Prague for free, if they are able to prove their age:
National identity card issued by a European Union country [ This proof cannot be used on trains.]
Passport [ This proof cannot be used on trains.]
A chip card registered in the PID Lítačka system with an application proving the age
A PID pass for Seniors from 65 years
If a senior cannot prove eligibility (age) by one of the above methods, they must purchase a full-price ticket.
https://pid.cz/en/discounts/#pid-pass