The proportion of new inmates at least 65 years old topped 10 percent for both men and women for the first time in 2014, according to the white paper on crime released Friday by the Justice Ministry.
Of the 21,866 people imprisoned last year, 2,283, or 10.4 percent, were senior citizens, the report said. That's the highest since data collection began in 1991.
By sex, female seniors accounted for 16.4 percent of all new inmates and male seniors 9.8 percent. The ratio of seniors among newly imprisoned females first topped 10 percent in 2009.
Recidivism among seniors is also a growing concern. Of those incarcerated in 2013, 18.1 percent were repeat offenders who had been released within the prior two years. But this figure was 24.9 percent among seniors, compared with 11.5 percent for those 29 or younger.
According to the report, the 274 seniors imprisoned in 1991, when data gathering began, made up only 1.3 percent of the year's total. That has grown almost every subsequent year.
On the bright side, the overall number of new inmates has declined for eight consecutive years since peaking at 33,032 in 2006.
Regarding rape, the white paper said that 50.9 percent of rape suspects last year were relatives or someone formerly acquainted with the victim, up 28.3 percentage points from 20 years ago.
As for indecent assault, 26.8 percent of suspects were family members or previous acquaintances in 2014, up 18.2 points from 20 years ago.
A ministry official said the rise in the figures reflects improvements in support for the alleged victims, which has allowed more cases to be reported.
Out of the roughly 1,800 people who were actually found guilty of rape or indecent assault in a one-year period from July 2008, 13.6 percent became repeat offenders. That ratio was higher for jobless people, versus students or people who were employed.
While 47.3 percent of rapists were 29 or younger, 51.3 percent of molesters were 40 or older, the paper said.