Umbrella Academy’s Klaus has the same powers as the Ancient One. Well, Ben is a ghost yet stillages. Klaus has the ability to astral project. The Umbrella Academy comics force other people out of their bodies. Well, Doctor Strange has a list of powerful monks. Ben, unfortunately, becomes a ghost following his death. Klaus was able to summon Ben’s living spirit.
The Umbrella Academy season 3 gives you the reasons why Ben seems to age over the years. In season 2 also Ben appears to age. So now you know Ben is a ghost and he should be stuck in the exact state he was in when he died. He had grown up alongside Klaus. Though he was a ghost he matured as an adolescent and that showed as if he were alive.
Ben’s ghost has been simply summoned afterlife. Klaus has a close relationship with his brother. That astrally project Ben’s spirit before it fully passed on. That forced him out of his body. That is what pushed the spirit into our plane of existence. The ability however came easily to the Ancient one. That is what we call the ability to move in and out of her own body.
Avengers: Endgame used it again. That pushed Bruce Banner’s spirit out of the Hulk’s body. That is what revealed its true self. You must know that Astral spirits seem to be able to interact with the physical and spiritual world to some degree. But there is someone who is controlling the projection. It is somewhat unlike traditional ghosts.
Astral spirits remain connected with their living bodies. Mirror changed their physical selves. All these qualities are displayed by Ben in the Umbrella Academy. That all made its way into the world. He is more like a living person than a traditional ghost. He is more alive. Also, he is more dynamic. Ben even changes his clothes.