It seems like it would be relatively easy to test out. People donate blood all the time. I'm sure the effects are known of how much blood you can replace at once without causing issues. Without a doubt some elderly people would volunteer. Why can't we start giving them new blood once a week from donors, and see the results on their health or illnesses? I'm all for testing, but this seems like one of those things you could quickly move to human trials.
Should be done outside the normal blood donor system. There's not that much donor blood around, so it would be unethical to use it. Not a practical problem, but it would look and feel bad.
Anyway, yes, why not try this on humans? Sounds perfectly reasonable.
Well, many people seem to be suggesting (including the researchers themselves) that there is a possibility of the older person dying of insta-cancer after receiving the blood because their cancerous stem cells would immediately be told to start multiplying. That's why I think it's a good idea to test this in animals first.