We have a six year old teenager. She likes to wear jewellery and her clothes always have to match. I pick out her clothes every morning, but sometimes it's a battle as she wants to wear a skirt with tights instead of pants. She likes her hair done a certain way and she likes to wear lip gloss and do her nails. I don't let her wear lip gloss that often though because our four year old likes to wear it too and she puts too much on, which can get messy. Usually, they only wear nail polish for a special occasion like a birthday party or during the summer. Some of the things our six year old says make her sound older. For example, my sister and my niece were visiting on the weekend and our six year old was going to the basement. My niece asked her where she was going and our six year old said "figure it out." We all started laughing. She didn't say it in a rude way, but it sounded funny coming from her as she sounded like a teenager.
Where do young children learn these things? I think children are exposed to external influences such as the media, advertising and stereotypes. I think our six year old sees a lot of the older girls at her school and wants to be cool like them. In my opinion, children are growing up faster than they did when I was younger. Our six year old has told me more than once that she wants to be older, and I always tell her to enjoy being a kid and not to worry about growing up too fast.
I think the best thing for parents to do is to always communicate with their children. Protect them from bad influences, but at the same time don't shelter them too much and explain things to them as they get older.