Is it normal to have vertigo when you're only 18?

No. It is more common as people age... However, it is certainly not "normal". Healthy 18 year olds should not have dizziness. Potential diagnoses: migraine, meniere's, medications, viral infections etc... If you have spontaneous spinning sensations, please see your doctor. Ent and neurologists and especially neurotologists are the people who specialize in this disorder.