Polish border guards say about 156,000 men aged between 18 and 22 entered Poland from Ukraine during the final four months of last year.
The guards say the figure represents cumulative entries, meaning one person could have crossed the border multiple times.
The border crossings by young men come after the Ukrainian government allowed men in that age range to leave Ukraine beginning in late August.
Up until then, most men aged between 18 and 60 had been barred from leaving the country, as a general mobilization order was issued in response to Russia’s invasion.
The government says the easing of the ban is aimed at giving young people opportunities to study abroad so they can contribute to developing Ukraine in the future.
But Oleksandr Hladun, an expert on Ukraine’s demographic issues, said this measure undermines the demographic and economic potential of the country.
He added that some young people will return in the future, but it is unclear how many, warning that this could have grave consequences for the country’s future.