When I became a dad a few years back I ended up watching a lot of PBS content for kids. I actually really enjoyed Arthur a lot. The art has a nice style, the characters are swell, and it tends to irritate me much less than other kid shows. But for whatever reason my sons (both < 6yrs) adamantly refuse to watch it. Maybe the look is for younger kids but the content is for older kids?
I probably would have watched it at that age, but then again, apparently I really liked watching the world news on the local PBS station at 6 years old. I haven't seen Arthur since the 90's, but my guess is it's probably better for kids between 8 and 12. It wasn't really my favorite show, but I do appreciate how it presents morality to kids.