http://www.raileurope.com/special-deals/rail-discounts/promotions-for-youth.html?WT.mc_id=CJ.youth_railpass.affiliates

If you or anyone in your party is a traveler below 26 years of age on the first date of travel, discounted rates are available for passes and tickets on some trains. These Youth rates are generally offered in 2nd Class only, dependant on availability within the specific country of travel and good for any traveler above 15 years of age.