the same thing happened in the next generation. Wilhelm's older brother Frederik was still crown prince of Denmark when his second son Carl became King Haakon VII of Norway.
In France the Grand Dauphin, son of Louis XIV, was still alive when his second son succeeded in Spain as king Felipe V. Monseigneur and his oldest son both died before Louis XIV so it was his grandson who became Louis XV.
In Russia Michael Romanov was elevated to the throne at the age of 16 with both his parents still living but forced into religious life. I believe his father had even fled to country in the time of troubles not to get killed only returning after his son became tsar. In fact it seems the father was the power behind the throne in the first years of Michael's reign.
In 1848 Ferdinand I of Austria abdicated and was not succeeded by his brother but by his oldest nephew Franz Joseph.
I heard that Prince William of Glücksburg, second son of the future King Christian IX of Denmark, was actually enthroned as King George I of the Hellenes before his own father was enthroned as the Danish reigning monarch. I already knew that the man we know in history as King Haakon VII of Norway was enthroned before HIS own father, the future King Frederik VIII of Denmark.
Are there any other examples in history of sons who acquired thrones before their own fathers? The example of King Michael of Romania doesn't really count, since his father (the future King Carol II) supposedly renounced the throne -- only to take it away from his son later on.
If a son became a sovereign before his father, it obviously would have to have been through the branching out of a royal house (we have discussed the Habsburgs, Bourbons, Coburgs, etc). This situation would be very unusual, and very different from a younger son reigning before an older son.