I agree that in principle you could filter out the ‘bad’ elements and add the ‘good’ ones, in reality we don’t know either the bad ones (except perhaps CCL11 and some pro-inflammatory cytokines) or the good ones (apart from GDF-11 and oxytocin). There may be molecules such as miRNAs contained in exosomes that are vital for age-determination – and yet others that we have no idea about.
I would check my math however – while the formula you use for the replacement of plasma is correct the number you get is wrong. About 1.5 plasma volumes would get you down to around 20% old plasma remaining after a single exchange. One and a half plasma volumes is fairly normal for a plasma exchange. So after two exchanges you are left with ~ 4% old plasma remaining. And that’s not a lot. Imagine that we knew all the factors needed to be changed- my guess is there would be hundreds and the costs would be tremendous.- you’d sill need plasmapheresis to accomplish that – so aside from allergic reactions what would be the advantage – I can easily see the disadvantages? And plasma is used under life-threatening conditions on the battlefield to save lives – without overmuch worry about allergic reactions.