science

They're talking about stretching out the whole ageing process. So if you live longer you also grow old more slowly.

Rats average lifespan is 3 years, but that doesn't mean they are really healthy, and then just drop dead, they go through the ageing process at a hyper-accelerated rate. Equally if humans had a lifespan of 120 years the old age part would move to around the 100 years mark, you wouldn't start getting old at 60 and then just be geriatric for the next 60 years.