Don't get too used to them.
Not everyone in 'House of the Dragon' is going to remain the same throughout the show. "Around halfway" the first season, several characters are going to change drastically, up to a point when they will be portrayed by different actors.
Among these characters is the main one – Rhaenyra Targaryen, who is now portrayed by Millie Alcock. However, at some point in the show, she is going to grow older, and will be reprised by Emma D'Arcy. The casting here is truly outstanding: you can actually see how similar these two are, looking exactly like a younger and older versions of the same person. We can only applaude 'House of the Dragon' casting team's efforts: they really did a great job.
Another such character is Alicent Hightower. The daughter of Otto Hightower will also grow up, with the older version to be portrayed by Olivia Cooke. As of now, we have seen Alison reprised by Emily Carey.
Young Laenor Velaryon will also see some changes. Currently, the character is played by Theo Nate. Later in the season, his older version will be portrayed by John Macmillan.
Last but not least, Laena Velaryon – the unsuccessful wife candidate for King Viserys, whom he dumped in favor of Alicent Hightower. The younger Laena is played by Savannah Steyn – who, unlike her 12-year-old character, is much older and is actually 22 years old. The older version will be portrayed by Nanna Blondell.
It's unclear when exactly the actors will change in the show and whether there will be a massive flash-forward to explain it. Given the vague "around halfway" hint from the showrunners, it might happen at some point around episode 5, with the first season consisting of ten episodes.
'House of the Dragon' is currently streaming on HBO Max, with new episodes arriving every Sunday.