Since Gap now sells clothes for adult women in size 0 and 1, either people have gotten smaller, or they are calling a 0 what used to be called a 4...if so then what used to be called a 14 is now a 10.

But as others have said, manufacturers differ. And it depends whether the clothing style is pitched at teens, who are still wearing shirts extremely tight. My 17 year old daughter and her friends buy shirts in the little boys dept.