Ienzo's a scientist but you know what he also lives in the same world as Mickey Mouse and Donald Duck and Goofy and all sorts of other talking animal shenanigans. He wouldn't bat an eye at a talking tiger so he can probably see Hobbes.

EDIT: Also if that doesn't work on the right kind of logic, depending on whether or not one considers Ienzo and Zexion to be the same person he's either eight with eleven years of somebody else's memories or nineteen but eleven of those years were spent in a weird emotionless mental state so. Schrodinger's maturity??