He is supposed to look like an old man.

Theon has been tortured repeatedly by Ramsay Snow, who has since been legitimized by King Tommen as Ramsay Bolton. Ramsay and the Bastard's Boys remove the skin on several of Theon's toes and fingers, leaving him in agony for days before removing the joints but only after Theon begs him to remove them. He breaks and removes several of Theon's teeth because Ramsay hates his smiles. It is also implied that Ramsay has removed Theon's genitals as well. Theon is forced to take on the guise of Reek, the name of Ramsay's late serving man. He is forbidden to bathe and is caked from head to toe in filth and excrement.

Due to the torture endured at Ramsay's hands, Theon looks like he has aged forty years. Besides missing teeth, many more are loose so he cannot eat without suffering agony. He has lost three stone in weight, his skin has turned pasty, and his hair is white and brittle. He can no longer use a bow due to the loss of three fingers, has difficulty riding, and he hobbles like an old man due to the loss of several toes.