Japan, the only developed nation to register more deaths than births in a year, will have senior citizens as its largest consumer group by the year 2020, Mitsubishi UFJ Financial Group Inc said in a report.

"The elderly will play a leading role in Japan's domestic consumption," the report said.

People in their 70s and older will account for about 20 percent of consumption, up from 13 percent in 2005, the Mitsubishi UFJ report said.