My friend always talks like a baby? She is 11 years old?

If she asks me something she will go down into a hush voice and mumble in baby talk. Everyone else in my group is growing up and starting to take new things on such as makeup, fashion, and listening to real world music (not hannah montana). How can i subtly hint (she is a sensitive person) that she needs to grow up now and that people dont take her seriously because of her actions. Help, ten points to best answer