What is the topic of the passage?

Imagine a world in which getting fitted with a new heart, liver or set of kidneys, all grown from your own body cells, was as commonplace as knee and hip replacements are now. Or one in which you celebrated your 94th birthday by running a marathon with your school friends. Imagine, in other words, a world in which aging had been abolished.
   That world is not yet on offer. But a semblance of it might be one day. Senescence, the general dwindling of prowess experienced by all as time takes its toll, is coming under scrutiny from doctors and biologists. Suspending it is not yet on the cards. But slowing it probably is. Average lifespans have risen a lot over the past century, but that was thanks to better food, housing, public health and some medicines. The new increase would be brought about by specific anti-senescence drugs, some of which may already exist.
   This, optimists claim, will extend life for many people to today’s ceiling of 120 or so. But it may be just the beginning. In the next phase not just average lifespans but maximum lifespans will rise. If a body part wears out, it will be repaired or replaced altogether. DNA will be optimized for a long life. Add in anti-aging drugs, and centenarians will become two a penny.