It's impossible for an old cell to become young again, at least with our current level of technology, because that would require you to reverse all the mutations that have built up over time in that cell. If your argument is that our bodies have many different repair mechanisms, and that as long as we can figure out how to activate them properly, we can then reverse aging, I would caution you that while you're correct in that repair mechanisms do exist, it's important to remember that these mechanisms are themselves composed from the same components as the rest of your body, i.e. they too will age and begin to fail over time.

The only way to actually cure aging would be to develop sophisticated enough outside mechanisms that would be able to intervene and replace these biological mechanisms as they fail. Macro level replacements won't cut it either. Replacing an organ might give you a few years here or there, but it doesn't address the issue that aging fundamentally happens at a molecular / atomic level.