Well, that's my issue, it wasn't a magical change that made him old based on how he should be chronologically (like black adam during the 52), his biological age because of him frozen was paused at whatever age he was frozen at. The serum kept his blood from freezing and he went into a makeshift suspended animation, so it's more that the serum indirectly kept him young. If he hadn't aged purely because of the serum I might understand but it was because of him going into the ice