I posted the bulk of this already in the Telomere Shortening section, but I'll restate it here with notes on how it applies also to the Brain Cell Nonreplacement obstacle.
The subject has a set of tailored symbiotes, specific to his own DNA and biochemistry, created and these are used to saturate the subject's bloodstream, repaired the accumulated damage of the Aging process. These symbiotes include:-
Proto-viruses which bond to the end of the subject's DNA strand, restoring the Telomeres and allowing the cells to continue replicating without any degredation.
Engineered Neuron Cells which repair any degraded brain tissue. The cells will bond to any braincells which display signs of aging, and proceed to cannibalise and replace the cell. The brain structure thus remains the same, but with a fresh, healthy cell in place of the old one.
When it first appears, around Biotech 5-6 in my opinion, it will probably be expensive to create the initial symbiotes. The expense and ease of administering the treament will go down with successive advances in rating until Biotech 8, when it will be possible to make the body manufacture it's own symbiotes constantly, creating Biological Immortality without any need for medical treatments.
I will add replies with additional clarifications, explanantions, amendments and classes of symbiote as people point out that they have become necessary.