When Queen Elizabeth dies Prince Charles will next be King but do you think he will pass it to Prince William as he is old himself?

He might if he was a member of another royal House. But the British custom, almost as old as its Monarchial system says: the King (or Queen regnant) on the moment of his/her death will automatically be succeeded by the Crown Prince. If the Crown Prince is at that time unfit to rule, a Regent will be appointed (which might well be Prince William). A King abdicating or refusing to take office has happened once (King Edward VIII, abdicating in 1936) and many Brits still consider it a scandal. The Queen is still perfectly capable of fulfilling her role at age 90; Charles is now 67, so he will have a good excuse not to step up only if his mother lives to be a 113 years old.