I'd go back to 15/16 year old me, show them the current PSP version of Tactics Ogre, and be like, "WHY THE HELL DID YOU GIVE UP ON THIS GAME?! :("
Seriously though, I got hooked on video games at 7 or 8...which was the mid eighties with the first Mario game. I'd show them a current version of Mario, and blow my 7 year old mind.