Most of us just assume that forgetfulness and senility come with old age. The truth, however, is that most older people don't experience a significant decline in their mental abilities [source: Zastrow].
Those who do experience a serious loss of memory or cognitive ability are usually affected by a degenerative disease such as Alzheimer's. And not all people get Alzheimer's -- about 1 in 8 people over the age of 65 has the disease [source: Alzheimer's Association].
There are more than a hundred other illnesses that can cause memory loss and apparent senility, but many of these are treatable [source: Zastrow]. So while you may forget where you put your keys from time to time, there's a good chance you'll remain sharp well into your golden years.