I am 18 years old and I have bipolar disorder. Will I have to be on medication for the rest of my life?

No one can say. Certainly medication is needed in most bipolar disorders. Severity varies, response to medication also is not always predicatble and the course of the disorder is probably the most unpredictable, especially in adolescents and children. Like anything else in life you are wisest to deal with things like that on a day to day basis. There are no guarantees about the future and no cystal ball.
Probably. Unless they find a way to fix the genes that cause it but u r fortunate to have meds that control the illness. Few meds fix illnesses most control them (hypertension, diabetes, hypothyroid, etc).