But exactly why do all these systems start breaking down? I agree we're not getting particularly close to 120 year lifespans with our current approach, which is tinkering with treatments for the ailments of old age. But I suspect there actually is a simple magic bullet somewhere - something to stop us getting old.
I doubt it. Nature doesn't work that way. Getting to 120 is going to take a LOT of engineering. You are going to have to manipulate the immune system in a fundementally more complex way than we're doing know. You will need to have better organ transplantation and you're going to have understand the brain. You're going to have to understand human biology at a much deeper level that we currently do. And you will be swamped with details.
Aging isn't just one thing. It's the pileup of a lifetime of little things going wrong until the bridge collapses.
You may be able to DELAY aging with some sort of magic bullet but that is likely to have a whole raft of unintended consequences. Not to mention, you're going have to start on it when you're about 20 years old.