Probably around the age of 9 or 10.
the whole tooth will die and fall out eventually
The front molar teeth fall out but the very back ones keep growing and if you loose the very back ones then you will never get them back. You will need False Teeth.
Definitely not normal. Sounds like some serious tooth disease happening there. See a dentist.
About your preteen age. You have twelve molars in total. The one near your canine and after that molar can fall out. Adult molars come out eventually.
I'm pretty sure they can.