i still had baby teeth when i was about 13 and then the dentist decided that the best thing to do was to take them out. very painful but worth it. not sure about how normal it would be for them to be left in till late teens though
It's more common that you would imagine. Some people have some baby teeth remaining all their lives. If a complete second set of teeth don't form in the gums, then there's nothing to push the baby teeth out - it's a simple as that. If the baby teeth are healthy they do no harm whatsoever.
after naomis answer feel i should add to my own, my adult teeth had come through but they had grown behind my baby teeth and so not pushed them out, i wasn't left toothless after having the baby ones taken out!
the only thing a dentist has done to my teeth apart from looking is taken one baby tooth out because the adult was comingthrough but cracked the baby but the baby wasn't budging.
most my classmates have got braces or fillings or both. so i'm doing quite well.