I found out about this about a month ago and I should have posted it up earlier... oops.
For a limited time, to celebrate the 30th anniversary of the TGV, you can purchase a 12-25 discount card even if you are aged between 26-30 years old. The card is valid for one year and non-renewable. It costs 49€.
Get in quick while stocks last! Limited to 400,000 pieces.