I was watching Buffy as it aired and never paid attention to their age, but always assumed there were like 27 years old (my memories before the end of season 5 are not great). I was surprised when Willow went back to the university, I thought "Wait what? She's still at the university?". Then when I did my first rewatch, I understood Buffy was 20 in the gift, and I thought: "Wow, that means they are younger than I thought then!"
I think this stroke me because all people around that age I know still live with their parents. They have their own life, jobs, are very mature but don't live alone yet.
I know two girls who are engaged (one just turned 20, the other is 23) and they still live at their parent's so I still see them, you know, young.
Oddly, I tend to see people who don't live with their parents as "more adult" in the sense they start a complete new life outside of the familial sphere.
And in Buffy, at 20, none of them lives with parents, which is why, following my weird logic, they look older. I don't know personaly any 20 years old boy who already lives in a appartment and has a good, well-paid job. Like I said I know people who are engaged, but not married at 20. Buffy lost her mum and has to take care of Dawn so it's natural that she really ages because she's thrown into full adulthood.
Willow and Tara moved into Buffy's house.
What I'm saying is that to me, people are fully adult when they don't live with their parents anymore. And I won't call myself an adult as long as I live with my mum. Because to me, being adult and being mature are two different things. Being adult means that you have to take care yourself of paying your loan, your bills, etc and that you are not supervised anymore by the parents.
So at 20, the Scoobies were adults to me, and I thought they were older because I don't know a lot of people who are adults (according to my idea of adulthood) at 20.
And some actors aged. Xander looks much older in the last seasons. Willow still looks very young.
But I always have troubles with the ages in series. Like when I found out they were 16 in the first season of Gossip Girl I was like "They are only 16?!"