Here in western Japan, Onomichi Prison, a small facility with a special ward for older inmates, who make up 22 percent of the prison’s population, is in the vanguard in dealing with this new problem. But recent visits to two large penitentiaries, one maximum security and the other minimum, underscored the more deep-rooted problems associated with the increase in older prisoners.

A recent Justice Ministry report said that older people were increasingly turning to crime out of poverty and isolation, suggesting a breakdown in traditional family and community ties. With nowhere else to go, more of the older inmates serve out their full sentences, instead of being released on parole like younger prisoners. What is more, recidivism is higher among the older inmates.

“There are some elderly who are afraid of going back into society,” said Takashi Hayashi, vice director of Onomichi Prison. “If they stay in prison, everything’s taken care of. There are examples of elderly who’ve left prison, used up what money they had, then were arrested after shoplifting at a convenience store. They’d made up their minds to go back to prison.”

While the main reason behind the explosion in graying lawbreakers is the rapid aging of Japan’s population, the rates have far outpaced the increase of older people in the general population.

Between 2000 and 2006, while the total population of Japanese 60 and over rose by 17 percent, inmates of the same age group swelled by 87 percent. In the country’s 74 prisons, the proportion of older inmates rose to 12.3 percent in 2006 from 9.3 percent in 2000, while the share of those in their 20s declined and in other age groups remained flat.

Japan’s rates are much higher than those in the West. America’s prisons — where those 55 years and over are categorized as elderly — are also graying. But such prisoners accounted for only 4.6 percent of the total prison population in the United States in 2005, according to the federal Bureau of Justice Statistics.