Yes, you will need to have a guarantor (typically a parent) co-sign your lease, even if you are over 18 years. This is because students typically do not have a source of income to cover their rental payments during their full-time studies, and even if they do, they typically do not have enough credit history to enable them to sign a lease on their own.
The situation is very similar to getting a co-signer for a mortgage. It ensures the landlord will get paid if you are not able to pay your rent. In the majority of cases, a student’s parents act as the guarantor. Therefore, if you do not pay your rent, expect your landlord to call your parents. Nobody wants that to happen, and the in the vast majority of cases, there’s no need to contact the guarantor because rent is paid on time.