Daemon's age in the show is not explicitly revealed, but neither are we led to believe that his age is significantly different from what it is in George R. R. Martin's books. So, let's agree that he is (give or take) 16 years older than Rhaenyra. This places him in his early 30s when we first see him meet his niece.
Rhaenyra's first marriage is to Laenor Velaryon, while Daemon's second wife is Laenor's sister, Laena. Daemon is, of course, younger than Rhaenyra's father Viserys, but he's still significantly older than her: in the books, there's a 16-year age gap between Rhaenyra and Daemon.
In 120 AC at the age of thirty-nine, Daemon married again, this time to his niece, Rhaenyra Targaryen, the twenty-three-year-old Princess of Dragonstone.
Milly Alcock, who plays young Rhaenyra, is 22. As of the mid-season time jump, Rhaenyra is aged up to 27, then—as of episode 8—to 33. Emma D'Arcy, who plays the older Rhaenyra, is 30.
Just like young Alicent, Rhaenyra is 14 at the start of the show and ages up to almost 18 years old over the course of the first four episodes. Milly Alcock was 21 years old when "House of the Dragon" season one was filming.
Milly Alcock, Rhaenyra, is 22 in real life. She's being replaced by Emma D'Arcy who is 30. The time jump in episode 6 is supposed to be 10 years, so Rhaneryra should be around 29.
"This is essentially a grooming scenario," Emma D'Arcy told The Independent. By House of the Dragon's fourth episode, Rhaenyra is around 18 years old and Daemon is 34. "The idea that a teenage girl is in any way able to consent to that sexual interaction is a mess.
The story of the princess and her valiant knight did not end as a fairy tale, though. A sordid and controversial story began to spread that Rhaenyra lost her virginity to her uncle Daemon at age 16. No one knows if that is true. But Targaryens had long married within the family to keep their bloodlines pure.
Rhaenyra is the only surviving child of King Viserys and Queen Aemma Targaryen. Following the death of Baelor, who only lived for around 10 hours in episode 1, Viserys declared Rhaenyra the rightful heir to the throne. She's a young woman when the show begins, just 14 years old.
Princess Rhaenyra is Daenerys' distant ancestor in the Targaryen family tree. Princess Rhaenyra and Daenerys are distant relatives, living 200 years and about eight generations apart, as this thorough Targaryen family tree demonstrates.
Daemon's relationship with his first wife, Lady Rhea Royce, comes about as an arranged marriage at the behest of his grandmother Queen Alysanne when he is only 16 years old.
Rhaenyra Targaryen, Played by Milly Alcock and Emma D'Arcy
Rhaenyra is 14 years old at the beginning of House of the Dragon and grows up to 18 during the first four episodes. Milly Alcock, the young Rhaenyra's actress, was 21 when HOTD season one began production.
In the seventh episode of the blockbuster HBO series, Prince Daemon marries his own niece, Princess Rhaenyra. The incestuous romance had been building for some time. Before a 10-year time jump occurred in the show's sixth epsode, Daemon took a young Rhaenyra to a brothel in King's Land where the two got intimate.
Aegon III, aka Aegon the Younger: Rhaenyra and Daemon's first child together. Aegon Targaryen, aka Jon Snow: Yes, he comes much later, when a song of ice and fire plays out. But he could be the prince that was promised, though that's still up for debate.
Daemon shows his genuine care and attention to Rhaenyra several times before the two marry. This doesn't take away from the many destructive and inappropriate parts of their relationship, but the books imply that Daemon does love Rhaenyra.
The duel that became known as the Dance over Harrenhal ended in the deaths of both riders and both dragons over the God's Eye. Daemon was 49 when he died.
The Targaryen family tree gets a bit complicated due to the incestuous practice of marrying sister and brother, but Rhaenyra is something like Daenerys' great, great, great, great, great, great, great grandmother. Daenerys (Emilia Clarke) is the Mad King's daughter with his sister-wife Rhaella.
39-year-old Smith (and Fabien Frankel too) have admitted they had no idea how old their characters were supposed to be in their scenes, and in an LA Times interview last Friday, Smith was pressed on Daemon's secret to eternal youth. It turns out his answer is: "He's always eating his greens. Gets lots of sleep.
One account says that Daemon seduced his niece and “claimed her maidenhood,” while the court gossip Mushroom's version of events has Daemon giving Rhaenyra “kissing lessons” and the like so she could in turn seduce her white knight, Ser Criston Cole.
According to the accounts of Septon Eustace, prior to her turning sixteen, Rhaenyra lost her virginity to her uncle, Prince Daemon, after he seduced her.
Acording to "Fire & Blood"... "Childbirth exacted a toll on the princess; the weight that Rhaenyra gained during her pregnancies never entirely left her, and by the time her youngest boy was born, she had grown stout and thick of waist' the beauty of her girlhood a fading memory...." And this was before marrying Daemon ...
Daemon couldn't perform with Rhaenyra after she took control of the situation. “It's the idea that Daemon is using Rhaenyra as a way to get at her father,” showrunner Miguel Sapochnik said.
In the books, Daemon and Criston are essentially the same age and Rhaenyra is about 15-16 years younger.
Jace is the oldest son of Rhaenyra and Laenor — or maybe of Rhaenyra and Ser Harwin Strong, depending on who you ask. Jace is currently the heir to the Iron Throne after his mother, and a strong kid despite only being somewhere around 7 years old.
Everyone who was a teenager in House of the Dragon Episode 5 has now been recast in Episode 6. Rhaenyra, Alicent, Laenor, and Laena are all being played by older actors to show they're all grown up now and ready for their own Game of Thrones.