Until recently, I assume even a human with an anti-aging mutation would've been killed by something else within a typical lifespan. War, disease, lung damage from wood burning stoves or coal, injuries while working, and so on.

Also, if it takes 4 different mutations (assuming each of 4 injections is a different substance), it might be very unlikely for all 4 to happen in one individual, and any 1 mutation may not give that individual any particular reproductive advantage.