Does anyone have any experience travelling on Italy trains using the "Junior fare 20%" on the trenitalia website?

Here's what it says :
"Junior: 20% for young people up to 26 years.
On board trains the Socio (member) tickets must be shown together with the Cartaviaggio, issued in the name of the traveller and an identification document"

Can someone tell me what the Cartaviaggio is? Or how does one access the junior fare?

Fodors has this to say :
"Travelers under 26 can get the same deal with a Cartaverde (Green Card). Those under 26 can also get discount fares under the Billet International Jeune (BIJ) and Euro Domino Junior plans."

How does one get the Cartaverde?