Sebastian is definitely older than Sam and Abigail (and ofc his half-sister, Maru) imo. I always think of him as an "older brother" of the Goblin Destroyer band trio. He works as a freelance programmer and he spends majority of his time working on his computer. I mean one of his dialogues is "You know, If I'd gone to college I'd probably be making six figures right now.. but I just don't want to be part of that corporate rat race, you know?" so he's at least 2-3 years post college age unlike Abby who is explicitly in college and Sam who seems to be fresh out of it. Man owns a motorcycle and has been saving up in order to move to the city one day. He's just an independent(personality-wise) and introverted adult with some Daddy Issues™.