How do you know if a child is mature?

Question

Kids ussually want cell phones to show to their friends so they look "grown up". I think that makes them more immature because it's silly to give an eight year old a cell phone-what's she gonna use it for? Or when they refer to kiddie things as "baby stuff" because of peer pressure. I think behaving like a young adult means being able to sit down with adults and have a serious conversation occasionally without acting up, ect. It goes both ways though: I think 8 year olds should still be going to chuckie cheese and the park.