I'm 18 years old and I have a tooth in the back of my upper left side thats growing out of my gum. Is that normal?

Possibly. It is normal, at yout age, to have a wisdom tooth erupt and expose itself. See your dentist for an x-ray confirmation.
Yes. 18 is a very typical age for wisdom teeth to start to emerge into the oral cavity.
No. A tooth growing out of the side of your gums instead of the normal position is not normal. Based on your age, the tooth could be a wisdom tooth that may be require removal.