One thing to know is that if you cannot remember something that you know that you used to know, whether its the name of your 2nd grade teacher, or who starred in your favorite movie, etc. but it later comes back to you, it is only normal aging and not Alzheimer's or another dementia disorder. If you forget something and it is gone forever, then it might well be early signs of dementia.
I use a little mental tool sometimes to remember something I am sure I still know. Just the other day I was trying to remember the first name of a man I knew when in the Army, a Brigadier General who was a genuinely nice guy (I was a Lieutenant Colonel at the time) who dropped dead while on a morning run. At first I could not recall his name despite my knowing him fairly well. I then start to recite the alphabet in my head, thinking of each letter in turn as the first letter of the name I am trying to remember. In this case I didn't have to go far, saying "A", then "B" and then the name Billy came into my mind (which was his actual name, not William).