IIRC it was during the opening credits.
I’ve just finished watching it.
There is a scene just before the opening credits which certainly refers to S&M with a girl - apparently a princess as she is referred to as ‘Your Highness’ - but there’s absolutely no indication she’s underage.
With a fictional series, it’s dangerous to speculate, but in real world Britain at the moment, there are actually no underage Princesses bearing the title of Her Royal Highness - the youngest is HRH Princess Eugenie of York who is now 22, but who was 21 at the time the episode was first aired.
(Technically, nine-year-old The Lady Louise Windsor, daughter of the Earl and Countess of Wessex, is probably entitled to be addressed as HRH Princess Louise of Wessex, but her parents have chosen not to have their children use royal titles in an attempt to give them a better chance at a more normal life).