AST September, a 55-year-old man lit some scrap paper on fire and threw it into a Seoul subway car as he left the train.

He had just been cursed at and kicked by senior citizens for sitting in a seat designated for "the elderly and the infirm".

The man, whom we know only by his surname of Kim, was sentenced on Jan 14 by a Seoul court to one year and six months in prison.

One news article reporting the results of his trial garnered more than 1,000 comments in just one day, most of which were from sympathetic younger people complaining about being forced to give up their seats on the subway to senior citizens. Kim is hardly young, but his frustration resonated with the younger generations.

The Seoul subway's designated-seating section has become a curious backdrop of intergenerational conflict in South Korea.

In the 40 years or so since full-scale industrialisation began, the social divide between generations has widened. Senior citizens grew up during Japanese occupation and the Korean War, and lived through the era of breakneck economic growth that followed, building a modern country from the ground up in just a few decades, most of the time under a military dictatorship.

Most younger South Koreans have come of age in a time of relative affluence and freedom, and like many younger people in East Asia, have gradually become more independent-minded than their elders and less attached to the traditional Confucian values that have been the basis of Korean society for centuries.

In recent years, South Korea's economic woes have put strain on both groups, and frustrations are high.

Older South Koreans are finding themselves financially unprepared for retirement, while younger people cannot find jobs. The Seoul subway is a rare place where the generations cross paths - and the intergenerational tensions are playing out in the crowded trains.

South Korea is one of the world's most rapidly ageing countries. It is expected to become an "aged society", according to the United Nations' definition of 14 per cent of the population above age 65, in four years. South Korea also has the third lowest birth rate in the world.

The country's elderly spent their lives assuming their children would care for them in old age and did little to prepare for retirement. But their children do not appear to be fulfilling their end of the bargain - and now the elderly are not faring well economically.

The relative poverty rate among senior citizens in South Korea is 49.3 per cent, the highest of the industrialised countries. Public pensions tend to be small. And the suicide rate for senior citizens, surely an indicator of economic strain, is the highest among the industrialised countries, at about 80 for every 100,000 people.

Meanwhile, South Korea's younger people are facing unemployment not seen since the early 1980s. More than half of the country's college-educated young people in their 20s remains unemployed or has stopped searching for work.

The notoriously congested Seoul Metropolitan Subway, which opened in 1974 and covers a total of some 320km, transports an average of seven million passengers per day, which is more than twice the daily average of London's Tube.

For years, "pushmen" were employed at busy stations to stuff hundreds of extra passengers into each train. In 2008, the subway switched to employing "cutmen" instead, who prevent people from boarding cars once they are full.

At both ends of each subway car, 12 seats are set aside for "the elderly and the infirm". As the name suggests, these seats are intended for senior citizens as well as for the handicapped and pregnant women, but in practice they are mostly occupied by the elderly.

According to the Seoul Metropolitan Subway authorities, there was a sharp increase in the number of complaints over the seats, from 252 in 2009 to 536 in 2011 (the latest year for which statistics are available). Anecdotal evidence suggests the trend has continued.

About two years ago, I had unintentionally sat in one of the elderly-designated seats on the subway and was checking my e-mail messages when I looked up to meet the eyes of a scowling elderly man. I got up right away. He did not thank me, but continued to stare at me from across the train. There had been other free seats for him to use, but he pressured me to get up just to make the point that I should not have been sitting there.

All subway lines in South Korea are free for those over 65 years of age, so most elderly people use it whenever possible. There was a time when young people were happy to give up their seats for the elderly. And the elderly, fitting to tradition, have always assumed they had a right to a seat occupied by a younger person. But young people today are simply less deferential to their elders.

The fighting over seats mirrors a vast political gap outside the subway.

A majority of older Koreans support President Park Geun Hye and the governing party, but the younger generation is strongly opposed to her leadership. Many older people feel nostalgia for the days of Park Chung Hee, the current president's father, when they were more prosperous and the country was in the throes of exciting development.

For now, South Korea's intergenerational conflict seems limited to the underground. But without a meaningful dialogue on how to help both our struggling elderly and disaffected young people, the tensions will find a way of rising to the surface.

NEW YORK TIMES