The Flag of the Netherlands
The Dutch National Anthem is Het Wilhelmus. It was written sometime in the 1500's and has been sung at official events since 1568, but was not adopted as the national anthem until 1932.
It is therefore the oldest national anthem since it was written so long ago.
The music is from a French Catholic soldiers' song called "Another song about the city of Chartres under siege by the Prince de Condé". The music was written down in 1626 by Adriaen Valerius and was also slowed down so it would be able to be performed in churches.
In 1815, the House of Orange, which was the monarch of the Netherlands, adopted "Those in whom Dutch blood" as the national anthem.
They didn't use Het Wilhelmus as it was the song that was associated with the House of Orange and they wanted a more neutral song for the country.
Even then Het Wilhelmus remained a popular song and was even played at the coronation of Queen Wilhelmina in 1898. On May 10, 1932 the national anthem was changed to Het Wilhelmus.