Russia is faced with a declining birth rate and a rising death rate:
On the plus side these population problems made Russia less of a military threat to its neighbors. For centuries Russia (rebranded as the Soviet Union in the early 1920s) was considered a threat to its neighbors by virtue of its larger population. But since the Soviet Union fell apart in 1991 (and half the population broke away to form 14 new nations) the remaining Russian population has been in decline. Twenty years after the Soviet Union collapsed, the Russian population implosion was getting worse. While in the 1990s the population was shrinking at a rate of .1 percent a year, in the first decade of the 21st century that increased to .2 percent a year. This was because the non-Slav Russians are having fewer children, just as the Slavs have been doing (or, rather, not doing) for decades. The Russian population has declined three percent since 1989, from 147 to 142.9 million. The proportion of the population that is ethnic Russian (Slav) has declined from 81.5 percent to 77 percent in that same period. The Russian slide could have been worse had it not been for the fact that millions of ethnic Russians in the 14 new states felt unwelcome with government controlled by the locals, not Russians in far off Moscow. Often the locals wanted the ethnic locals in their midst gone and Russia made it easy for ethnic Russians to return to the motherland. This prevented the Russian population decline from being closer to ten percent. Until the recent invasion of Ukraine, sanctions and lower oil prices, the Russian birth rate was growing. That has stopped.
Russia still has the reputation of being a country willing to lose 30 million people--as they did in World War II--to win a war.
But the reality of modern Russia is far different. The biggest question may be whether Putin could force the Russian people to accept high casualties to win a war.