[XorNot]

More over, babies can clearly grow from limited cells to "young" versions of fully differentiated human tissues. Which means from some initial stock, you can replace the vast majority of the bodies cells with younger versions - i.e. with plausible, attainable technology we would generally expect to be able to grow immunologically identical replacement organs and major tissues. That definitely is an engineering problem, more then anything else.

The only truly troubling one is the brain, and we're very much not sure if it actually is one or for example, suffers degradation from the degradation of the body its attached to - likely both - but we also know that the brain is not a static structure, and so replacement or rejuvenation of key systems would definitely be possible (certainly finding any way to protect the small blood vessels in the brain would greatly help with dementia).