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.