"Memory isn't perfect even when it's working at 100 percent efficiency," says David Wolk, MD, assistant professor of neurology at the University of Pennsylvania's Penn Memory Center. That means glitches like these tend to be normal, and if you're in your 40s or 50s the chances of having a serious problem are rare. Less than 5 percent of people get Alzheimer's before age 65, and it's even more unlikely prior to 50. These little slip-ups aren't even necessarily indications that you're headed for trouble in your 70s or 80s. (See 11 Signs of Serious Memory Loss) Read on to learn why some forgetting is normal and what you can do to keep your mind as strong as possible.