By Linda Sieg and Ami Miyazaki

TOKYO, April 19 (Reuters) - Their youth long gone, members of Japan's generation of "parasite singles" face a precarious future, wondering how to survive once the parents many depended on for years pass away.

Some 4.5 million Japanese aged between 35 and 54 were living with their parents in 2016, according to a researcher at the Statistical Research and Training Institute on a demographic phenomena that emerged two decades ago, when youthful singles made headlines for mooching off parents to lead carefree lives.

Now, without pensions or savings of their own, these middle-aged stay-at-homes threaten to place an extra burden on a social welfare system that is already creaking under pressure from Japan's aging population and shrinking workforce.

Hiromi Tanaka once sang backup for pop groups, and epitomised the optimism of youth.

"I got used to living in an unstable situation and figured somehow it would work out," Tanaka told Reuters as she sat at the piano in a small parlour of an old house connected to her elderly mother's next door.

Now aged 54, Tanaka relies on income from giving private singing lessons to a dwindling number of students, and her mother's pension to make ends meet. She has no pension plan of her own, and has used up most of her savings.

"My father died last year so pension income was halved," she said. "If things go on like this, my mother and I will fall together."

Tanaka is one of the growing ranks of "life-time singles," whose numbers hit a record in 2015, according to data released this month that showed that among 50-year-olds, one-in-four men and one-in-seven women were unmarried.

"During the 'bubble economy' until the mid-1990s, the 20-somethings were happily amusing themselves. They thought by the time they were in their 30s, they'd be married," said Masahiro Yamada, a Chuo University sociologist who coined the term "parasite singles" in 1997.

"But one-third never married and are now around age 50," Yamada told Reuters in an interview.

FRAGILE FUTURE

The trend is not only a factor behind Japan's low birth rate and shrinking population. It also puts an extra damper on consumption since new household formation is a key driver of private spending.

And since about 20 percent of the middle-aged stay-at-home singles rely on parents for support, they also threaten to weigh on social safety nets. "Once they use up inherited assets and savings, when nothing is left, they will go on the dole," Yamada said.

The rise in those shunning marriage, experts say, is due not only to more diverse life-styles but to an increase in low-paying, unstable jobs. Part-timers, temps or contract workers now account for nearly 40 percent of the workforce compared to about 20 percent in the 1980s.

Although recent tightness in Japan's labour market has meant a slight fall in the number of singles living off parents, the overall trend probably won't change, said Katsuhiko Fujimori, an economist at Mizuho Information and Research Institute.

"That's because of the increase in irregular workers and the fact that more and more people cannot marry for economic reasons, even if they want to," he said.

Some middle-aged singles living with parents once had steady jobs but slipped off the career track due to illness or corporate restructuring as companies cut costs to compete.

"Once you fall off the regular employment ladder, it's tough," said Hirotoshi Moriyama, a member of a non-profit organisation that tries to help middle-aged people find jobs.

LEFT BEHIND

Akihiro Karube, 53, worked in the advertising business after graduation and by his 30s was earning a hefty salary. He moved back with his parents after a short-lived marriage but paid his own rent until, aged 43, he was diagnosed with Parkinson's disease and had to quit.

Efforts to find work as a qualified home helper for the elderly have failed and he says he now relies on his father's pension and a disability pension of his own.

"I just wish I had a stable income, that's the main thing," said Karube, who lives with his widowed 84-year-old dad in public housing in a Tokyo suburb.

The future looks especially bleak for an extreme sub-set of people who not only live at home with their parents but also seldom venture out, living out their days in hermit-like seclusion. Known in Japan as "hikikomori", and once stereotyped as mostly young men, these stay-at-homes are also ageing.

Fuminobu Ohashi was one himself, but now he works with a support group that last year began holding workshops for parents worried about their offsprings' future.

"The problem is what they will do after their parents pass away," Ohashi said. "It is a quietly ticking time-bomb." ($1 = 108.6100 yen) (Editing by Simon Cameron-Moore)