He's not. He was born in 1982, in Romania.
Mark Hamill has three children, who are indeed more or less Sebastian's age, but Sebastian is not actually one of them.
What I see on the Net shows Max Lloyd-Jones acting parts of it, and a CGI - de-aged Mark Hamill doing voice/face shots.
https://winteriscoming.net/2020/12/23/who-played-luke-skywalker-the-mandalorian-season-2-finale/#:~:text=Mark%20Hamill%20returned%20to%20the,of%20the%20Jedi%20Luke%20Skywalker
Quote:
The credits for the finale lists actor Max Lloyd-Jones (
War of the Planet of the Apes) as the “Double for Jedi.” Presumably, he played Luke during the bits when the Jedi Master was mowing down Dark Troopers, that being a bit much to ask of the 69-year-old Hamill.
Still, he wasn’t the only person involved. Mark Hamill
was actually on set for filming and did more than just provide Luke’s voice. He shot Luke’s conversation with the group, with VFX technology used to make him look like the younger Luke we remember from the original trilogy. That’s some impressive work right there, although YouTuber
Shamook thinks it can be improved upon. Is his deepfake video even more seamless? You decide:
Hamill
tweeted that managing to keep this scene secret was “nothing less than a miracle,” and we have to agree. “A real triumph for spoiler-haters everywhere!” But of course, the show began its existence with a big reveal that didn’t get out beforehand. “We started off the whole series with the Baby Yoda reveal – with the Grogu reveal – that was a secret that everybody was really blown away by,” Favreau told
Good Morning America. “Part of that is not telling too many people, not having merchandise that’s being tooled up and announced and everything. And then the very last episode having Mark Hamill actually on set filming and us using whatever technology we had available to de-age him and try to make him look as much like he did in the old films.”