1) There's no real "borderline" between Mädchen and Frau. Women can be called Mädchen for quite a long time even when they are grown up. Young women in a sports team are often called Mädchen instead of Damen or Frauen even when they are around 30 years old. On the other hand as soon as they have finished school and begin to work (sometimes at the early age of 16) you would officially call her Frau. Very often there is used a kind of mixture in addressing young women, especially when they are younger than 20: They are addressed by their first name and the formal form "Sie" instead of "du". Men are addressed in the same way of course.
2) I'm sorry, but I'm not sure if I got your second question right... Anyway - Fräulein isn't used anymore. Sometimes, when you speak to a very little girl, you would say Fräulein to her when you are angry with her, like "Jetzt ist aber genug, mein Fräulein!!!" (That's enough now, lady!). Many years ago a waitress or a shop assistant was usually addressed as Fräulein. And some older people still do so sometimes, but that sounds really unfriendly. But these are the only situations when it's used nowadays.
3) I believe that it was like a spontaneous feeling in the public. Equal rights of women are a very important item in Germany. If there were just a few extreme women lib groups insisting to change things like that, it wouldn't work at all.
4) A child, whose sex isn't known, is named "es" (it) which is neutral. That's because "das Kind" (child) is a neuter word in German. A "Junge" (boy) is always named "er" (he) because "der Junge" is masculine. "Das Mädchen" (girl) is neuter, but you can say "es" (it) or "sie" (she) when talking about her. Grammatically correct is "es" of course, but colloquially "sie" is used even more often.