Seoul (CNN) In South Korea, teenage gangs aren't necessarily a threat. Increasingly, it's the elderly you need to watch out for.

That's according to official statistics, which recorded a 45% increase in the past five years in crimes committed by people age 65 and over. Serious crimes including murder, arson, rape and robbery rose 70%, from about a thousand cases in 2013 to more than 1,800 in 2017.

In one case, in November, a man in his 70s was arrested for allegedly assaulting a courier over a late parcel. When police arrived, it emerged the man had forgotten he'd already received the package two days earlier.

In August, another septuagenarian allegedly killed two civil servants and injured a neighbor over a water dispute. And in April, a 69-year-old woman reportedly poured pesticide into a fish stew due to be served at a village event.

Gray crime wave

More than 14% of South Koreans are over 65, making the country an official "aged society" under a United Nations classification.

Yet while they are living longer, many cannot support themselves financially as they age. About 60% of elderly Koreans do not qualify for the national pension, which was not introduced until 1988 or made compulsory until the late 1990s, and in 2017 half were living in relative poverty.

"With no jobs to allow the elderly to contribute to society, they feel disconnected and this can lead to animosity toward others, depression and antisocial behavior," said Cho Youn-oh, a professor and criminologist at Seoul's Dongguk University.

"Isolation and feeling that they have nothing to lose could lead them to lose control and behave recklessly. People with more connections to society through family and jobs tend to have more self control -- that can stop them from (committing crimes)."

A view of Seoul's Nambu Correctional Institute, which houses a large number of elderly prisoners.

Senior prisoners

Even the country's prisons are struggling to cope. Senior prisoners can bring with them a wide range of health problems including dementia, cancer and kidney issues -- and often must be segregated from the rest of the population.

"Not only are they physically weaker than the younger ones, when they're kept with younger people, the chances of them getting into fights are higher due to the generation gap and cultural differences," said Lee Yun-hwi, deputy director of Seoul's Nambu Correctional Institute.

CNN visited Nambu's #2 wing, where the prison's elderly population lives -- evidenced by a wheelchair, scales and a device to measure blood pressure in the common area.

A typical Tuesday morning on #2 wing starts at 9 a.m. with aggressively cheerful music piped over loudspeakers. About 30 elderly inmates dressed in blue two-piece uniforms and white shoes move to the auditorium for an aerobics class.

An aerobics instructor leads elderly inmates in simple exercises.

As a song titled "What's Wrong With My Age" plays, the instructor urges her charges to move from side to side, bending and kicking their legs. Their actions are slow -- but for many of the inmates, who spend most of their time in small cells, it's an important part of the day.

"It's good for preventing dementia, and I think it's emotionally healing," said Park, 71, whose full name and crime have been withheld to conceal his identity.

Park, who has been in Nambu for two years, believes the spike in elderly crime is the result of a lack of jobs and support for the aged. "The crime rate goes up when people have no money," he said.

Parts of Nambu Correctional Institute can resemble a hospital wing more than a prison.

Another inmate, 70-year-old Noh, wishes South Korean society took better care of the elderly.

Noh was born in the late 1940s amid huge chaos and instability in Korea, as the peninsula was liberated from Japanese occupation before plunging into civil war, leaving millions of famiiy members separated and hundreds of thousands orphaned.

He said his generation had suffered through some of the hardest times in Korean history -- and yet was left without savings or the support of wider society.

Elderly inmates make small bags in their spare time to earn a little bit of cash they can receive on their release.

Searching for a solution

Rejoining society, of course, represents a significant problem for many inmates. About 30% of elderly convicts commit crimes after release -- above the general recidivism rate of 20%.

Criminologist Cho said a social support network could make all the difference in preventing future offenses. With South Korea on its way to joining Japan as a "super-aged society" as early as 2025, he said the public must understand the difficult situation that many elderly citizens face so that such services and policies can gain broader support.

For now, one of the most secure places for many elderly prisoners may be prison itself. When many inmates are released, Noh said, "they have nowhere to go or sleep. No money for food to eat."

While he counted himself among the lucky ones, with a wife and children on the outside to support him, "prisoners who spent 10 to 15 years inside are afraid of being released, because they have nowhere to go."