Most people granted workers' compensation for mental ailments were in their 30s, a survey by a health ministry-affiliated group says.

The Health, Labor and Welfare Ministry reveals the tally and causes of compensation cases every year. But the ministry-linked Japan Organization of Occupational Health and Safety further analyzed data from January 2010 to March 2015 to get a better picture of karoshi, or death from overwork.

The survey covered about 2,000 mental illness cases caused by long hours and power harassment, and about 1,500 related to heart and brain diseases. The results were revealed to a group of karoshi victims' kin on Tuesday.

According to the data, all claimants had worked an average of 99.6 hours of overtime a month before diagnosed with a disease.

It also showed that karoshi-linked suicides claimed more of the nation's younger workers than did heart and brain diseases.

"It suggested the need to place greater weight on measures to tackle mental health for younger workers," the group said.

According to the survey, the average age for developing mental illness is 39.

For work-related heart and brain illnesses, including heart attacks, most were in their 50s, with symptoms first surfacing at the age of 49.3 on average.

In 31.6 percent of the cases, symptoms of mental illness began to emerge in their 30s, followed by 26.6 percent in their 40s and 22.4 percent in their 20s.

For heart and brain diseases, 36.7 percent began suffering in their 50s, followed by those in their 40s. Only 18.9 percent cited headache or heartache problems before that time, with over 70 percent saying they hadn't experienced any symptoms, it said.

The organization plans to further analyze the data by industry and investigate cases in which workers applied but weren't granted workers' compensation.