Britain's Prince Philip still acts like he's 55.
Peter Phillips says his 94-year-old grandfather was "sensible" to put his health first and pull out of the Battle of Jutland commemorations last week but he is "as determined as ever" to keep going with his royal duties.
He said: "Yeah, I think he is OK, it was probably a sensible decision given the weather and all the rest of it. But I think he, you saw him, he was out and about yesterday.
"He's 94 years-old. His mind still wants to do what he did when he was 55-years-old. So, he's alright ... He's nearly 95-years-old but his head thinks he's 55-years-old, he's as determined as ever to plough on and to keep things as normal as possible."
And Peter, 38, who is the son of Princess Anne and her first husband Captain Mark Phillips, insists Queen Elizabeth and Prince Philip have no plans to slow down.
He added: "I don't think it's a secret that they are both looking to reduce their workload rather than increase it and, you know, the younger members of the family are definitely taking on a lot more of the workload. Certainly, the duke is now at the point where he only wants to do things that he enjoys."
Meanwhile, it was previously revealed that Prince Philip was forced to pull out of the engagement on the doctor's orders.
A royal spokesperson said: "The background is that this is on the doctor's order, but there are no plans for him to go to hospital. This is a temporary health thing."
Whilst a source added: "Its commemoration is very important to the duke. He is generally very fit but for a man of his age a bad cold can easily lead to something worse, like pneumonia. Travelling on a plane, with air conditioning, is not ideal."