Prince Charles
has proved himself on many levels, though. He has already proved to us that he is an intelligent, thinking, well-read, progressive person.
I think he will be 75-80 when he is king, and given that he is probably one of the healthiest people on the planet, he could well live past 90. I hope for 100, though, at least!
Prince William could be about 50-55 by the time he is king, a good age, as he will doubtless have a family and be exceedingly mature and intelligent like his dear old dads. If he could pick up his father's healthy habits and quit the smoking and drinking, it will be nice too.