Why were students under 17 not allowed for Triwizard Tournament in Harry Potter and TheGobletofFire?
Because in the Harry Potter and the Goblet of Fire the Ministry of Magic, Dumbledore, and Barty Crouch wants that Harry must not take part in the tournament and make Harry safe. But happens just opposite of it, Barty Crouch Junior ( son of Barty Crouch ) made it happen on the wish of Voldemort, and Harry Potter was chosen as the second champion of Hogwarts in the Triwizard Tournament. Later he was used to bringing back Voldemort as he was the Horcrux, unfortunately, Cedric Diggory was murdered by Voldemort. I think that Dumbledore was knowing that Voldemort would try to come back with the help of Harry Potter, so Dumbledore with the help of the Ministry of Magic make the rule to save Harry Potter from Voldemort and any other dangerous factor. But he was failed to do so.