Here’s an answer to what I think is behind your question. I think you are assuming that the brain, by dint of its function in progressively accumulating information (whether facts, skills, predilections), will be intrinsically un-rejuvenatable because restoring its structure to an earlier age would by definition erase memories. But we don’t need to restore THAT aspect of its structure. We can totally leave the strength of synaptic connections intact, while we clear out the plaques and tangles and so on, and any senescent glia, and replenish lost neurons, etc.