If young blood works, wouldn't young bone marrow work better? After all that's what creates the blood. Fuck this piecemeal crap I'm signing my kids up for bone marrow donation (for me). Go one step further and if my son is a similar height to me when he's 18 then I'll get a femur transplant from him, I'll give him my good one in return, I'm not a monster.
I joke obviously, I have no wish to extend my life span. But if it worked I can see people having children for just such a purpose, much like having a second child to find a compatible donor for a first child with an illness.