What happens if you're 15 and haven't started your GCSEs?

It would depend on the person. There was a girl at my school who was moved down to my year as she was ill through most of her Y10 and so she started GCSE's with us. Again, if it is a person from abroad they would also go to Y10, I know people who have done this, as it really would be almost impossible to catch up. Another option would be to go to college and do your GCSE's there.