According to these figures the median age is already (almost) as bad as it gets: https://www.statista.com/statistics/624303/average-age-of-the-population-in-germany/

Not considering regional effects, like lack of immigration into federal states with large amount of neo nazi population. They will be fucked more thoroughly.

https://en.wikipedia.org/wiki/Selection_bias

the government is going to pay that back, plus interest.

Minus inflation.