South Korea's Population Pyramid
population.un.org
population.un.org
They drop their restrictive immigration policies and let people in; or
They reduce their minimum age to work. Embrace child labour; or
They increase the retirement age greatly; or
Accept the consequences of societal collapse in population decline. SK already dealing with silver crime.
So Germany's decision was immigration. When yugo collapsed and eu formed, germany stole a ton of people from the balkans. They accepted a crazy amount of people from the arab spring and syria. and now ukrainians. Not to mention open door to practically anyone.
North america is debatably the one behind events like the arab spring with hopes to bring those people here.
Not the only thing, one of the big deals was retirement funds are allowed to go bankrupt now. Quite unpopular to increase retirement age, instead you basically just let their retirement funds die and they naturally get forced back to work.