a) In a way that's economically viable.
And there's a reason I said "in descending order" - each is only a concern if the one(s) before it aren't practical.
You're right though, my list was not exhaustive.
a) In a way that's economically viable.
That depends very much on how much protein you need and how much people you can treat. It's possible to spend 100 million to genetically engineer yeast to be extremely optimized for producing a certain protein. A big pharma company can spend that much money when the drug is a block buster drug but researchers who just want the protein to run a few experiments can't.
In experiments performed on mice, blood transfusions from young mice reversed age-related markers in older mice. The protein involved is identical in humans.
http://mic.com/articles/88851/harvard-scientists-may-have-just-unlocked-the-secret-to-staying-young-forever