haha ryan, you and your trepanation fetish.
to further elaborate on what deep has mentioned in regards to alzhiemer's...it is true that a post-secondary education can ward off/postpone it, but that it is generally genetically determined.
however, even those who are predisposed may be capable of retaining memory if they keep their minds active...there was a study, (no source, sorry,) done where it was proven factory workers and those who do monotonous tasks on a daily basis for a period of decades were more inclined to become senile later in life. those who were actively challenged in their professions, or even read in their spare time and constantly sought new intellectual experiences for themselves, were not as likely to suffer severe cognitive degeneration.
essentially, i believe, everything you have ever learned is still locked away, potentionally inaccessible in your brain. various theories have been postulated stating that it may be that new information crowds out the old, as well as a few others which i forget.
(a university lecture on the "persistence of memory" five years ago has escaped me.)
most people only remember the most relevant information that they are exposed to that is neccessay for day-to-day functioning.
for example, phone numbers are approximately 7-digits because that is all the average mind is capable of retaining.