The number of people with dementia aged 65 or older reached an estimated 4.62 million last year, accounting for 15 percent of the age bracket, according to a health ministry survey.

A study group under the Health, Labor and Welfare Ministry also calculated that another 4 million senior citizens, up from 3.8 million in 2010, suffered from mild cognitive impairment, which can evolve into dementia.

When these two totals are combined, the estimates suggest 1 in 4 people aged 65 or above are experiencing at least some symptoms of senility. Further, the survey, released Saturday, projects that 15 percent of senior citizens — or 4.62 million of the 30.79 million people in that age group in 2012 — have dementia.

Takashi Asada, a University of Tsukuba professor specializing in dementia, and other members of the study group collected data on 5,386 senior citizens in eight municipalities nationwide by interviewing them and their families between fiscal 2009 and 2012.

As of 2010, the number of people aged 65 or older with dementia was estimated at 4.39 million. Of this total, 2.7 million lived at home, including around 430,000 who lived alone.

By age, the number of people with dementia aged 74 or younger stood at 10 percent or less, but the figure topped 40 percent among those aged 85 or above. In most age groups, more women developed dementia than men.

In earlier projections last year, the health ministry calculated that the number of elderly with dementia rose to 2.8 million in 2010 and to 3.05 million in 2012, based on data collected on those in need of government-backed nursing care services.

The latest estimates far exceed those figures, underlining that many senior citizens with dementia do not use services provided under the public nursing care insurance program.