How long you can remember a memory it depends on how you input an specific memory in your mind. For example, if we plinth a pillar deeply then it'll take more times to root up in a constant effort. Similarly, if we take a incident or subject or anything seriously it'll take more time to forget.

Now why we forget old memories? Though our memory can be expanded Infinity but it's normally limited. Our memory power increases or decreases with time and pressure we put on it. Brain doesn't erase old data but it erases unnecessary data or comparatively less important or which we don't recall for a long time to make room for new ideas and data. At the old age brain fails to remove junk data and usually can't make any room for a new one, that's why a old men less sensitive to remember something.