First of all, a good thing you are thinking of fasting at age 18. Fasting will help you with all sorts of health issues in your life.
Your body is designed to eat food reserves and fats when fasting and not muscles. If you starve yourself then you will deplete muscles. This happens when you have consumed all the reserves and scar tissues that body uses for fuel. If you reach that stage after 30 plus days, your body will choose some lean muscle for fuel.
Yes, while fasting exercise is not recommended. Body heals better when resting and in warm state.
I have been in Ketosis in two days and a few times in four days, so it is not always 3 days of fasting that gets you in Ketosis.
Fast away my friend, but read about it before you do. Water Fast means nothing goes in your mouth but water.