Forgetfulness and dementia are different things.

Individuals in their forties may experience “forgetting things”, for example, not being able to remember the names of people, what they had for lunch the other day, or what task they were suppose to do. As forgetfulness increases, more and more people become concerned about the onset of dementia.
In Japan, the number of elderly people with dementia in 2012 was around 4.62 million (about 1 in 7 people over the age of 65 years), and this number is expected to reach 6.75-7.30 million individuals by the year 2025, which is around 1 in 5 people.
As mentioned above, “forgetfulness” is a decline in brain function that comes with age and not dementia. However, if the degree of “forgetfulness” is all together the experience itself, such as not being able to remember that a person actually met with someone or not remember actually having lunch, and so forth are common symptoms of the beginning of dementia. In this case, care and attention is required.
Although there are many cases where it is difficult to distinguish between age-related “forgetfulness” and dementia-induced “forgetfulness”, the difference gradually becomes clearer.

The following are the characteristic differences between dementia and forgetfulness:

Forgetting due to Aging	Forgetting due to Dementia
Forgetting part of an experience.	Forgetting the experience all together itself.
Realizing that you forgot something.	Not knowing that you forgot.
Forgetting WHAT you ate.	Forgetting THAT you ate.
Simple mistake when cooking.	Unable to judge how to season the cooking.
Incorrect day or date.	Not knowing the date or the day of the week.
Trying to locate a lost item.	Thinking the item was stolen.
Apologizing when a mistake gets pointed out.	Making excuses or getting frustrated when a mistake is pointed out.
Forgetting to buy an item while shopping.	Forgetting that you’ve already gone shopping, then going out shopping again.