1) Yes you become Baligh as soon as one is the signs show, even if it is before 15.
2) They have to count from the age signs of puberty showed. Not 15, but rather 11 or 12, the time they are sure they were baligh. They have to make up those prayers and fasting.
3) If you are not aware there is no kafara.
4) Yes if the fasting was missed intentionally and knowingly they have to pay kafara. The kafara can be paid whenever. Even many years later. The day that was missed must be made up too.
5) They can make up the fast whenever. It does not need to be done consecutively.