Old plasma dilution reduces human biological age: a clinical study

Young blood has good stuff in it. Adding young blood improves health.

Old blood has bad stuff in it. Removing old blood improves health.

(or both could be true, I suppose)

This study suggests that it's more "old stuff has bad stuff in it". They weren't adding any young blood at all. They were removing the plasma from old blood, replacing it with saline and albumen, and putting it back in. But they still saw similar effects to what other studies had seen with adding young blood.

It may be that there's "bad stuff" in your blood that can't be removed by the kidneys, liver, etc., and this "bad stuff" builds up over time.

No matter how good the oil filter on a car is, you still need to change the oil once in a while. :-)