Which is the youngest capital city in Europe?

Cardiff became a capital in 1955.

Cardiff did not become a city until 28 October 1905. And it was only proclaimed the capital of Wales in 1955.

https://www.bbc.co.uk/blogs/waleshistory/2011/10/cardiff_city_status_1905.html

The capital of Bosnia and Herzegovina is Sarajevo. Bosnia and Herzgovina gained independence from Yugoslavia in March of 1992. Berlin was made the capital of the unified Germany on October 3, 1990, which was also the day of re-unification. That is from www.germany-info.org, a German website.

I think, though, that Serbia and Montenegro beat out Bosnia/Herzegovina by a little over a month, gaining their independence on April 27, 1992. The capital is Belgrade, with Podgorica the judicial capital.

The emergence in 1993 of Slovakia becoming an independent nation makes Bratislava the newest capital.
http://www.slovakopedia.com/b/bratislava.htm

Berlin became the capital in 1990 when the unification of Germany was consummated with a state ceremony. Since then united Berlin has once again become the capital.

Chisnau or Kishinev, capital of Moldova in 1991, was never previously a capital.

Serbia and Montenegro split to form two separate countries on June 3rd 2006, making Podgorica the youngest (since Belgrade was already the capital of the former Serbia & Montenegro).

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