I watched SS regularly until I was about 9, and my mom didn't really seem too phased by it. Maybe it's because she also enjoyed watching it to a certain degree, particularly the videos we bought such as
Sing Yourself Silly and
Monster Hits. And the only time I remember getting teased by my peers over SS is once when I climbed into a trash can and said "I'm Oscar the Grouch", and one of my peers said "That's a baby show!". Of course, I never really talked about SS much with my peers.
I could kind of understand why parents of children in the 6-12 age bracket might be concerned about their child's development if they're still watching shows aimed at preschoolers. But I don't think banning SS is the answer - maybe figuring out why they still watch it would be a better idea. And once you get past your teenage years, I think the idea of being "too old" for a TV show becomes obsolete - at that point, it just means you're being nostalgic and/or getting in touch with your "inner child". Or it could just mean that you appreciate how they used to make SS enjoyable for adults as well.
I don't know exactly when or why I stopped watching SS. I don't think I ever specifically made a point to stop watching kids' shows, but I think it might have been a subconscious thing. It may have also had to do with the fact that the show was changing too much for liking.