Prince William urged his workaholic father to relax at age 70, to spend more time with his grandchildren.
The Duke of Cambridge, who has three children – five-year-old George, four-year-old Charlotte and four-year-old Louis – said, "It's something I'm working on harder," he says.
"I'd like him to have more time with the kids. Now that he's 70 years old, it's time to consolidate a bit because, as most families would, you're afraid you will not have them with you.
"And he's the best guy I know, but I want him to be in shape until he's 95, you know, he's going on.
"So having more time with him at home would be nice, and being able to play with the grandchildren. Because when he's here, he's brilliant. "
Charles admitted that he would have to change his habits of interference when he was king.
In a documentary celebrating his 70th birthday on Wednesday, he spoke for the first time of the sort of king that he could be.
Asked about his "interference" in areas such as climate change and planning, he replied, "I always wonder what the interference is.
"I mean, I've always thought it was motivating. I tried to make sure that everything I did was political. It is essential to remember that there can only be one ruler at a time, not two.
"You can not be the same as the ruler if you are the Prince of Wales or the heir. The idea that I will follow exactly the same thing, if I have to succeed, is nonsense.
Until he becomes king, Charles, the heir to the throne who has served the longest time in British history, will continue to make his voice heard.
But he said, "I'm not so stupid. I realize that it is a separate exercise to be sovereign. So of course you know that I totally understand how it should work.
"I will not be able to do the same things as my heir. Of course, you operate in … the constitutional parameters. "
Charles also urged his son William to find his own way when he will become the Prince of Wales.
He said, "Every one of my predecessors had to know how they would do it. Many of them died dead or were killed before going very far. "
A BBC crew followed Charles for nearly a year for John Bridcut's one-hour documentary, which will air on BBC1 tonight at 9 pm
Mr. Bridcut said, "He has incredible energy. He has so many balls in the air at once, it's amazing. "
The Duchess of Cornwall said that Prince Charles was so determined to succeed that it was like he "wanted to save the world".
Camilla described her husband as "a very exceptional man" who reads Harry Potter books in a funny voice to his grandchildren.
She said, "I would like a lot of people to see their lightest side."
But she acknowledged her dedication to causes such as the Prince's Trust charity, and that her work on the environment and climate change could be detrimental to her.
She said, "He feels all inside, that's why he's getting things done. He is quite impatient. He wants things to be over yesterday, I think everyone who works for him will tell you. But he is motivated by this passion to help him.
Prince Harry congratulated his father for intervening to walk his wife Meghan into the alley after his father was too ill to attend their wedding.
Harry said, "I asked him and I think he knew it was going to happen and he immediately replied," Yes, of course, I will do everything Meghan needs and I am here to support you. "
Harry, 34, also urged Charles to ease his workload. Harry says, "He really needs to slow down. It's a man who dines ridiculously late into the night.
"And then goes to his desk and fall asleep on his notes until he wakes up with paper stuck to his face.
"He never stops. I mean, when we were kids, there were bags and bags and work bags. We could hardly go to his office to say good night.