This is basically filtering the plasma to remove harmful stuff, which is present in the older mice but not the younger. That's probably because the older mice's liver and kidneys aren't as effective as they used to be. The conjoined experiment worked because the young mouses liver and kidneys were filtering for both of the mice.

This sort of filtering (which includes albumin replacement) is already available for humans. My ex-wife was hospitalized when her liver failed. Her blood was full of toxins, which overwhelmed her kidneys and eventually caused them to fail too. The toxins in the blood had a lot of bad effects, and would have killed her. While she was waiting for a liver donation, she had to undergo blood dialysis pretty much every day. She was hooked up to a machine that pumped her blood out, filtered it, replaced albumin (which was lost in the filter), and then pumped it back in. Each treatment took about two hours, and was really stressful and dangerous. Her blood pressure had to be carefully managed, and the machine couldn't keep her blood at the exactly correct temperature so it ached when going back in.

Bad as it was, it kept her blood clean enough for her to hang on until she got a transplant. She was on kidney dialysis for a year after that until she got a kidney transplant too.

This was incredibly expensive, because of the albumin. Also incredibly dangerous. You would not want to do this as an elective procedure.