A number of chemicals simply accumulate in the body. Cholesterol is quick to come to mind, but it should be noted that liver and kidneys accumulate a lot of chemistry that they don't get rid of. Indeed, a lot of body parts accumulate crud that over 70 years won't matter but will prevent you from reaching 700.
Again, for a Replicant, this is easy - just edit it out. NT6
For everyone else, its messier. Quite literally.
Biocycling everything save the brain using replicator generated organs strikes me as the 'easiest' way. NT6, BT6
Another is to change the biology. Inorganic cells anyone? NT9
Still another is to use the techniques for genetic resynthesis to drain off the accumulated chemicals. BT9
I don't see this as a problem that requires an in-depth solution. From the descriptors of the various Biotech levels in the rulebook it's clear that at higher levels the various organs are replaced with more effective and versatile versions. So by the time a civilisation's Biotech level has advanced to the point that chemical depositing becomes a significant obstacle to longevity the process of designing more efficient versions of the liver, kidneys and whatever else will be a simple one.
Remember, when seen from a scientific evolutionary point of view, the Human Body is a jury-rigged mess that has a great deal of room for improvement once we work out how to make these improvements without bringing the whole thing crashing down.