He's already 73 years old lmao, he doesn't have much time to "reign."
The Prince is even lucky to be alive by this time to be called king.
At 73 years he is lucky that it happened in his life time. The queen died at 96 years and ruled as much as the son. The king should count himself lucky actually.
"One of my lecturers once said that Prince Charles is the most frustrated Prince in the world because his mother has lived a very long life." - And a very happy child though

This is true , he may not have shown it but as a human nature people who know the history of the kingdom would know that he is heir to the thrown and finally it has come to past. And at the last he is now a happy child.