A few more thoughts:

  1. The brain only thinks when it has to This is why we can, for instance, touch type very fast. The brain doesn't consciously figure out what buttons to push, or where they are, so it doesn't remember it.

    When you are old, you don't have to think as hard to get through the day - it's mostly routine. Therefore, you don't remember it, and the day seems to fly by.

  2. The brain likes to optimize things Although this is somewhat anthropomorphic (Can you be anthropomorphic with the brain?), it is also true. If you do a certain task a lot, the brain will optimize things so that you can do it without conscious thought. Touch typing is one example of this. I don't have to think to type. I just focus on what keys/letters I want pressed.

The brain is weird.