There's no surplus of young men. Historically, Chinese men who did well had multiple wives, whereas those who were poor rented after they had saved up for it. Today, Chinese incomes are high enough that single men can import wives both from non-Communist (Indonesia, the Philippines and Thailand) and nominally Communist (Vietnam, Cambodia and Laos) neighbors, all of which now have lower average incomes. What's impressive about China is that per capita output has gone up almost 70x since its economic liberalization in 1979. It's not hyperbole to say that there has never been anything like this in recorded economic history. Which is why economists are so impressed.
The single child policy is probably the one thing that *might* hold China back from foreign military adventures. Because the one thing that might touch off a national insurrection is a war that kills off many family lines. And successful Chinese insurrections have, in the recent past, involved the partial extermination of the previous ruling class.
The Chinese are individualists to a fault. They don't get involved in politics for the greater good because the cost of losing in the Chinese milieu can involve getting killed perhaps along with their families (dispatched to labor camps to be starved to death). The only thing worth fighting for is your family. With one child families, the most important element is that child. A regime that puts the entire nation's only children at risk is risking its continued existence.
This is why the Party keeps raising the military budget (~$200b today) and is transitioning to an all-volunteer force.
Re elder care, the Chinese are pretty naturalistic about these things. They don't sent the elderly out into the wilderness to die of exposure as they used to, in times of severe deprivation. But beyond a certain point, they throw their hands up and cut off life support.
Liu Bang initially served as a minor patrol officer in his hometown, Pei County (in present-day Xuzhou, Jiangsu), under the Qin dynasty. Sometime in the 210s or 200s BC, he rebelled against the Qin government by releasing a group of convicts he was escorting to Mount Li to construct Qin Shi Huang's mausoleum, after which he and his followers became outlaws and took shelter on Mount Mangdang. In 208 BC, when rebellions erupted throughout China to overthrow the Qin Empire, Liu Bang formed his own army and participated in the insurrection. He styled himself "Duke of Pei" and emerged as one of the most prominent rebel leaders after taking control of Pei County and some adjacent counties. After the fall of Qin in 206 BC, Xiang Yu, the de facto chief of the rebel forces, divided the former Qin Empire into the Eighteen Kingdoms. He declared himself the king of Western Chu and appointed 17 former rebel leaders – including Liu Bang – as the rulers of the other kingdoms. Liu Bang became the "King of Han" and was given a domain in the remote Bashu region (parts of present-day Sichuan). Later that year, Liu Bang led his forces out of Bashu and attacked and conquered the Three Qins, three of the Eighteen Kingdoms which were nearest to his domain.* Late because the average lifespan in antiquity was 40+ years of age, which is why most men and women married in their mid-teens. Men who married late in life were usually individuals who could not afford the bride price. In this "surplus young man's" case, it's likely he never did come up with the money.