One of the great features of The Crown has been the fabulous attention to casting accuracy across all of the episodes involving not just the major royal family characters, but also the cast of English Prime Ministers and other major figures through the history of Queen Elizabeth's reign. This episode departs from that trend with the mistaken casting of William and Harry. In Season 6 episode 4 we see 15 year old William played by 16 year old Rufus Kampa and almost 13 year old Harry played by 13 year old Fflyn Edwards. Rufus is an almost perfect fit for the now awkward mid teen William. He would've seamlessly transitioned to 16 year old William in episode 5 Willsmania but the casting lept to 24 year old Ed McVey who actually did a fabulous job as the university aged Wills.
Similarly casting 22 year old Luther Ford as the 13/14 year old Harry in Willsmania was a massively inaccurate casting leap from Edwards who was perfectly cast as the early adolescent Harry easily covering his age in episodes 4 and 5 which was sad because Ford was excellent as the young adult Harry. These were very puzzling decisions given the very small age gap portrayed between episodes 4 and 5.