Thought experiment: let's say any and every component of the human body was transpantable (i.e. not just blood, kidney, liver).
Then if you transplanted a set of components X from a person of age 30 into a person of age 60, then the person's resulting age should logically be somewhere between 30 and 60, skewed towards 60 if X is small (one drop of blood) and skewed towards 30 if X is large (everything except the brain).
Now the question is: how do you calculate the "weight" of the set X that controls the skew? Is it something simple like number of cells or mass? Are some components preferred, such as heart cells or kidney cells?
Also, importantly, does the quality of "age" diffuse and equilibriate throughout the body, (like temperature), such that after some equilibriation period the whole body will be the same age? Or if you transplant a 30-year-old heart into a 60-year-old, will you just get a 60-year-old with a younger heart?
Then if you transplanted a set of components X from a person of age 30 into a person of age 60, then the person's resulting age should logically be somewhere between 30 and 60, skewed towards 60 if X is small (one drop of blood) and skewed towards 30 if X is large (everything except the brain).
No, you ignore the effect of the body fighting transplants because they have different DNA. The brain also learns to control different parts of the body when it's young and when you change those parts of the body in a way that makes them react differently to neural impulses the body get's less efficient.
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