I'm sure there are multiple factors, but you already mentioned the obvious one. 1990 and 2016 are 26 years apart.. in other words a generation apart. Less babies today because there's less women in their child bearing years today due to the drop in births from the previous generation. Generations echo across time. In the same way that millennials in the US are the largest generation because they echo the baby boom.

https://en.wikipedia.org/wiki/Russian_financial_crisis_(2014%E2%80%932016)

i think that plays a part, it was a huge financial crisis due to oil price crash, price crash for some other commodities that Russia exports, sanctions due to Crimea annexation etc.