Our cells age through accumulated DNA damage, and associated damage to repair mechanisms.
These processes are central to life, cancer and death. Fiddling with them might be hard, but we know what to look at, and that's a big step.
Some people still look at mitochondrial dysfunction, where mutations can stack up more quickly, but it seems that mitochondria do not limit age: a look at the number of functional mitochondria in an old person shows very little functional change at a cellular level.
Telomeres, which shorten with each cell cycle, can be lengthened, but apparently their length does not actually correlate with cell senescence or human age. That rather indicates that they're not central to ageing.
A combination of stem cell rejuvenation with DNA protection should allow us to live a very long time, except for neuronal problems.
Neurons are very long-lived cells, and replacement happens slowly. It would be much harder to maintain a brain, especially with slow build-up of problems in many cells at once, as in the diseases mentioned earlier, because we can't just take a load of cells out and then replace them.
Furthermore, if we do manage to rejuvenate brains, I suspect that it will involve some memory loss.
