The way I look at it, the older you get the more information your brain has in it so we don't have room to hold as much.
We only retain the more interesting or significant things.
When I was younger, drunk or sober, my memory was so sharp. Now their are so many things I have totally forgotten and I am sometimes so forgetful in my day to day life.
I am sure alcohol abuse has hurt my memory but I think it is mostly age. I know people that never drank and are totally air headed and forgetful.