Japan has the third lowest birth-rate in the world (only Saint Pierre, Miquelon and Monaco have lower rates, but they are very small, not even reaching 50,000 inhabitants combined). 2016 became the first year that birthrates dropped below a million since statistics began being collected in 1899. Birthrates have been steadily dropping since the 80's, and starting around 1990 the working population has been in decline as well, with a steady increase in the elderly.
In the post-war era (late 40's to mid 70's) Japan had a quickly growing population. Birthrates were high and it was common for people to have very secure employment. After college, people would often stay with the same company until retirement. Wages and job security were high, and costs of living were lower. These factors led to high birthrates. Today however, temporary jobs are much more common, and wages have stagnated, especially for the young. Many young people continue to live with their parents well into their 20's and 30's.
It is uncommon for the Japanese to share housing if they aren't married, unlike in the West where many young working people share housing to keep costs down until they are more financially stable. In addition, births outside of wedlock are extremely low, around 2% to America's 40%. Most Japanese wait to be married before having kids, however it seems that the young women are quite selective when it comes to finding a marriage partner. Most do want to get married according to surveys, but financial standards are very high, ruling out most men in their age-category, who are generally lower earners. So both men and women tend to stay single, and not have children.For those that do, the high costs of raising a child tend to keep families small.
Politicians have introduced some monetary incentives for having children, even creating a government agency devoted to increasing birth-rates. But these measures haven't been enough, with the high expense of childcare, resulting in many parents caring for their children on their own and unable to work. This goes against the strong work culture present in Japan, which even has a word for job stress-related suicide. With high costs of living and higher education, and no maternity or paternity leave, most enter their careers with high levels of debt.
On top of all this, unlike many other countries with low birthrates, Japan has basically no immigration, and small interest in changing this. While efforts to bring birthrates up have been insufficient, there is hope that improving technology could replace large numbers of workers, leaving the younger generations more free to have families . However these advances could take decades to achieve these hopes. By 2055, it is estimated that as much as 50% of the population will be over 65 or under 15, with the lion's share being over 65. This leads many to fear that the Japanese will become practically extinct.