Cody and Joe are twins but Cody was sickly when born. He spent the first year and a half of his life in hospital growing well enough to go home. When he came back, Joe was already walking and slightly talking. They had to lie to Cody about his age to hide the sickness.

Idk if this makes sense but basically this HeadCannon implies that Cody is the younger twin and he was sickly so he stayed in the hospital for a while, then when he came home they lied about his age.

Yeahhhh