I am 18 and have one baby tooth left. It is feeling wobbly. What will happen once it falls out?

Unknown. It is unknown what will happen unless a dental x-ray is done to see if there is a permanent tooth available to replace.
See dentist. If baby tooth over-retained, it means surrounding teeth do not meet properly as baby molars larger than permanent premolars that replace them. If no premolar present, teeth on either side and in opposing arch will collapse into the space left when baby tooth lost. Before this happens, see your dentist for evaluation. Don't be surprised if he recommends seeing an orthodontist.