That's starkly apparent looking at population pyramids. Russia's trails off tremendously with age, but has a large young population. Japan's is immensely top-heavy.

Russia: https://www.livepopulation.com/images/chart_age_russia.png

Japan: https://www.livepopulation.com/images/chart_age_japan.png

From:

https://www.livepopulation.com/country/russia.html

https://www.livepopulation.com/country/japan.html