I once saw an experiment with mice, where they connected the bloodstream of a younger mouse to the bloodstream of an older mouse. And the older mouse became "younger", or at least healthier.

If I could isolate the stem cells of a human and make a constant flow of them into an adult human, how much would I extend his life?

Or, when he reached what would be old age (70 to 100 years old), would that be pointless because mental illnesses like Alzheimer's would occur and he would consequently be more likely to stay in a vegetative state and consequently death?