Youth from 12 up to 25 years inclusive travel at a reduced fare on Eurostar to Belgium/France, TGV and trains from the Netherlands to Belgium via Roosendaal border (note: not on Intercity Brussels). On Eurostar and TGV the Youth fare is subject to availability.
Travelling around one or more European countries is cheapest with an Interrail pass. There are special fares for young people up to 27 years of age inclusive.