As the following x-ray shows, as we get older, and continue to use and reuse our brain cells, over, and over, and over, and over, and over, and over, and over again, those brain cells begin to get very smooth, and slippery, and warn out, and thoughts and memories simply can't stick there quite as effectively as they used to, so they just slip-slide away.
Actually the older ones get thicker and it takes more energy to create new ones and there are so many more to sort through. Like your computer when you have so many programs on it it will slow down.
You lose neurons that are not used or your mind thinks it will not be needed the next day. :)