Who was the youngest knight of the Round Table?

Probably Sir Percival.
This is the best answer I could find, since the ages of the knights are usually not specifically stated. But it also depends on what point in time you're referring to, since new knights were added as time went on while others, such as Tristram, died. At one point Sir Ector De Maris was the youngest (as stated in "The Champions of the Round Table"), but this was before Percival was knighted.