If one has a constant learning rate and a brain (or equivalent) of any given size then eventually the oldest memories are forgotten unless an effort is made to refresh them. If one decreases the learning rate then senility is a danger. As an alternative one can add extra memory - which in the long run means becoming posthuman (or postbiont, postvec, etc.) to manage all that information. Another alternative is to undergo periodic "childhoods" where the brain is made more plastic and one forgets a bit but not everything from previous incarnations, but this entails some loss of identity and some personality shift. Corporations like Psyber Retrievals, Inc - "Let us remember it for you!" - offer memory storage units for this reason.