How will we fund the nanobots?
Nanobots are self reciprocating. So after the first one, it's free xD
Or the machines produce billions so, nanobots would necessarily be cheap.
But any cure for cancer could be a way for biological immortality.
Although not eternal youth, that would be nanobots. Also, bulletproof skin etc. would be nanobots. Turning you connective tissue, like collagen in to graphene etc.