The prince said: "I've grown up, everyone's got to grow up. But there's something inside me, I'm always going to have that little sort of - how do you say? - child streak."
About wearing the Nazi desert uniform complete with swastika armband in January, the prince said: "Maybe it was a sign of my own immaturity. That was then, this is now."
"I am becoming 21 - something like that I will never do again. It was a stupid thing to do. I think it's part of growing up."
The interviews were conducted at the Prince of Wales' Home Farm, in Highgrove, Gloucestershire.
Prince Harry said the Duchess of Cornwall was "a wonderful woman and she's made our father very, very happy, which is the most important thing".