Many health experts say dementia is more common today because of longer life expectancies. About two-third of its victims are women, probably because they live longer.
Some of the common symptoms include asking the same questions repeatedly; becoming lost in familiar places; being unable to follow directions; getting disoriented about time, people and places; and neglecting personal safety, hygiene and nutrition.
Changes in personality and behavior can also be signals, particularly if it affects work or daily activities.
Two of the most common forms of dementia in older people are Alzheimer's and multi-infarct or vascular dementia. In the latter, a series of strokes or changes in the brain's blood supply may cause death of brain tissue and symptoms usually develop abruptly.
An estimated 60 percent to 70 percent of all dementia patients have Alzheimer's, which tends to develop as a slow progression, according to Neil Buckholtz, chief of dementias of the Aging Branch at the National Institute on Aging.
Thatcher likely has Alzheimer's disease because of the slow progression of her memory lapses, according to Buckholtz, who has not treated the former prime minister. Oftentimes, "pure" dementia and that disease can overlap and one exacerbates the other.
Alzheimer's disease can take as long as 20 years to develop, which led some to speculate that Reagan, who left office in 1988 and was diagnosed in 1994, could have had early signs of the disease in office.
"President Reagan was physically active and that may have contributed to a longer [development] time for him," said Buckholtz. Other behaviors that can slow the disease are staying engaged mentally and socially.
There is nothing to suggest that Thatcher, too, had early disease while in office, according to Buckholtz.
Her daughter first noticed symptoms during a lunch in 2000. The once razor-sharp Thatcher confused Bosnia with the Falkland Islands during a conversation about the former Yugoslavia.
"I almost fell off my chair," writes Carol Thatcher. "Watching her struggle with her words and her memory, I couldn't believe it. She was in her 75th year but I had always thought of her as ageless, timeless and 100 percent cast-iron damage-proof. From the fateful day of our lunch, tell-tale signs that something wasn't quite right began to emerge."
Doctors can never be completely sure whether the memory-related symptoms are Alzheimer's disease or another form of dementia, though an MRI test can give some clues. Only an autopsy provides the "pathological gold standard," said Buckholtz.
"Because prominent people do have Alzheimer's, it may be a surprise, but it shouldn't be," he told ABCNews.com "Although there is some indication that people with more education and higher education levels are at a reduced risk."
While some dementias are reversible, Alzheimer's is not and modern drugs only slow its progression.
"You lose your sense of self and it's very difficult on family members and caregivers who see a person who doesn't recognize them anymore," he said.
Meanwhile, the details of Thatcher's disease are being doled out in Britain's The Mail.
The author, Thatcher's now 55-year-old daughter -- a twin, whom the British tabloids portrayed as jealous of her more flamboyant and doted upon brother -- appears to have written her own "Mommy Dearest."