I found an article which claims that not only does Prince William get the interest on his inheritance upon turning twenty five, but also that once he's reached that age, he no longer has to get the Queen's permission to get married, lol. Here's the link to that story, for anyone who's interested:
Prince William Inherits, Harry still pinches pennies
*I don't know how true any of them are, but I thought them to be an interesting read. So I decided to share. Does anyone know if that "rule" about needing the Queen's permission to get married, is true?
William still needs permission and consent from The Sovereign and the Government to marry as the spare to the throne, although he is 25.
Although there is a mechanism under the RMA for him to marry without consent, the price would be high, including an Act of Parliament removing him from the succession and the loss of his royal title and rank.