I dunno, but really it depends on how well the actor ages onscreen. Like, for instance, Gabrielle Carteris, who I believe was 30 when 90210 started. The first couple of years, they could pass her off as a high schooler, but one day, it was like she aged 20 years overnight. I could say the same about Ian Ziering, but the guy looked 27 through the entire run of the show.
Never watched that show, so I don't remember that stuff.
Actually, Buffy's cast seems to be blessed with gracefull aging. Aly just turned 30, IIRC.
Anyhoo, I don't think the kids will grow up to fast. But I would be suprised if they did all the films- I would think that they would be worried about typecasting.