If you're over 18, can doctors prescribe different medicines for adults then minors?

Yes and No

Explanation:

Going off of the Nino2125's response, the answer is yes and no.

Yes in a sense that there are different medications and dosages associated with different age groups so you could possibly be prescribed some other medicine as you turn into an adult at the age of 18.

No in a sense that even if medication matches your age group, if the dosage/and or prescription is not fit for you based on many factors such as family history, other medication usage etc, you might not be prescribed a different medication necessarily. Doctors will most likely prescribe medication which is best for you