This was the link where I found the info
»foreigners over 70 years of age can travel for free on Prague public transport. To be able to travel in Prague public transport for free they need to arrange a special document issued by the Prague Public Transport Company in their offices listed on
http://www.dpp.cz/en/advance-sales-in-metro/ for which they need their ID (passport), a passport sized photo (with signature on the back side) and to fill in a short form to be able to buy the document for 20 CZK (ca. 1€). It takes ca. 5 minutes.It is not possible to use a passport to be able to travel for free.For more details contact the Prague