No, you cannot start the graduated licensing process in Ontario, Canada at age 15.

To apply for a licence in Ontario, you must be at least 16 years old and pass a vision test and a test of your knowledge of the rules of the road and traffic signs according to the Ministry of Transportation (MTO) . After you pass these tests, you will get a Class G1 or M1 licence and a driver information package for new drivers. 

Read our article that explains more about the G1 license of Ontario, Canada.