Growrh and development wouldn't play into this,I’m sure 18 YO blood would do the trick anyway. Especially if their adolescent development isn’t completely done. It’s the growth and development factor that counts right?
growth , development and such is hormonal, testosterone, estrogen, thyroid and pituitary hormones, can't get enough of that from blood donations, this would be more stem cell type experimentation I would think. And even then not sure it would actually make a differance without more " additives". This can't just be transfusions alone, that would be basically just blood doping, which while works is only a very temporary solution...
Not sure just what this is but its more than just young blood.