People who need extra time to memorize things may be excellent at solving complex, detailed problems. Intelligence rests on having a storehouse of information, and the ability to solve problems logically, not just the ability to quickly remember isolated facts. The terms dementia, senility and Alzheimer's disease are often incorrectly assumed to be interchangeable. Dementia is the medical term used for what historically was called senility - deterioration of intellectual capacity, commonly, but not necessarily, seen in older persons. It is more than just memory loss - it involves loss of memory, reasoning, judgment, calculation, speech and comprehension.
Many older persons have temporary lapses in memory, without suffering true memory loss.
This can be normal. This is normal. Older people take longer to commit something to memory, and longer to retrieve things from memory. This is normal. Dementia exists when a person experiences difficulty carrying out normal tasks as a result of deterioration in overall intellectual capacity.