In Birth By Sleep, Young Xehanort's body is completely covered by a hooded Black Coat with a fairly tall and masculine build. He never speaks at any point of the game.
In Dream Drop Distance, he has silver hair, dark skin, an adolescent face and golden eyes.
Young Xehanort's body model is a copy of Saïx's. It may or may not have a deeper meaning as Saïx/Isa has not transformed into a full clone of Master Xehanort yet.
During his battle against Terra, Aqua and Ventus he uses two glowing blue swords which are similar to Xemnas' Ethereal Blades. It can be combined into a dual-blade, and extend to be lashed out like a whip. In the first phase of his battle during Kingdom Hearts 3D: Dream Drop Distance, he uses the No Name Keyblade that can be use as a whip like Kingdom Hearts Birth by Sleep. During the second phase he sends out phantoms, similar to the clones he creates in Birth by Sleep that use his dual-blades.