Most research in life extension is based on reducing the incremental damage caused by radiation, stress, toxins etc. so they will increase the duration of youth

Most research in life extension is based on reducing the incremental damage caused by radiation, stress, toxins etc. so they will increase the duration of youth and middle age as well as feeble old age. I think a lot of the real world gains in life span are coming from entirely different things though, like treating and preventing heart attacks and cancer. This does end up extending the "old" part of our lives.