I have seen 17/20 year olds in tv (I think, I don't double check actress ages, but I'm sure at least in porn there are 18 year old ones) and I WOULD NEVER expect to see a show/movie about 11 year olds emulating that.
And you are basically saying (because it is in tv) we should accept the same behaviour from an 11 year old as an 17 year old, which I strongly oppose. there is 6 years difference and A LOT of things in between for me to find it ok. I don't mind a 17/18 old to be naked on tv, I don't think it is ok for an 11 year old. I think it is ok for 17/18 year old to emulate sex/erotic things on tv, I don't think it is for a 11 year old.
There is also the concept of consent, which I don't think an 11 year old should be able to give to spread her legs on tv, while a 17 year old should (maybe not all but hey...).