In 2019, Shulman et al.  summarized the known lithium therapeutic ranges for Old Age Bipolar Patients (OABP). For patients over 60, the range is 0.4-0.8 mmol/L, and for those over 80, 0.4-0.7 mmol/L. For younger patients, the standard region is 0.6-1.2 mmol/L. The current research leaves three disconnected therapeutic ranges. A unifying theory is developed explaining the therapeutic range of lithium for all ages. It explains why the changes in the therapeutic range happen and exactly how the upper limits change on a year by year basis instead of by decades. It also explains why and when the standard therapeutic range should be abandoned. In developing the theory, only information about the changes with age in the eGFR is used; when applied to lithium, the theory explains why the therapeutic regions for over sixty and over eighty exist. The known ranges were discovered using empirical evidence. The theory gives a more precise start for those regions as well as how to move smoothly through them. It applies for bipolar patients who do not have chronic kidney disease and shows that a new therapeutic region for those over 90 needs to be created. Table 1 is developed using the new theory to give the upper limit, the midpoint, and the twenty-fifth percentile up from the minimum level of the therapeutic range for each year from age fifty to one hundred. This table enables doctors to keep the patient’s serum level where they want it as the patient ages.