The youngest king in the world?

Answer 1

Alfonso XIII of Spain was declared king when he was born. His father, Alfonso XII of Spain, died 6 months before his son was born. Of course his mother Maria Christina of Austria was appointed regent during his minority. In 1902, on attaining his 16th year, the King assumed control of the state.

Answer 2

I'm not entirely sure what you want:

you ask about the world, but all your chosen categories are Britain-only. Also, you put it under history, but talk in present tense.

The youngest kings in history are those born posthumously and turn king at birth, like John I of France.

Answer 3

King Tuthankahmen at 14 years old