返老還童is nice dream, only a dream never the less.

I suspect the key here is the stem cells from the young mouse helped the old mouse. If that is true, it is very difficult to apply that  to human.

GDF11 they identified is a growth factor, one among hundreds even thousands of similar growth factors. It is possible that it has positive effect in reverse aging in some area, but it is not going to be the wonder drug people had dreamed for since dawn of mankind. And there is always side effects when use growth factors.

Basically it is my expert's opinion that we can slow down aging and extent human lifespan to some degree, but becoming a immortal or forever young is not possible.