The 25-year old teenagers are starting to look weird
In S1, you could just about accept that Steve, Nancy etc were teenagers, despite being in their 20s. But by S3, when the younger kids have started to become teenagers themselves, the older ones in comparison just look like those slightly-built adults that play teenagers, like Mickey Rooney and Judy Garland.
Steve's Scoops-Ahoy five year old kid's sailor suit doesn't help either.