It is true that standards for past drug use varies depending on the department. Many departments automatically disqualify you if you have used any illegal drugs other than what would be considered "experimental" during the last ten years. It sounds as though your use of marijuana was more than experimental. Having used marijuana six months ago would also disqualify you with most police departments. Below is an example of a past drug use requirement from one department.
Have no history of extensive illicit drug use, or any history of delivery or transportation of a controlled substance for profit, or be a "current" user of any illicit drug. "Current" use is defined as the use of marijuana or the abuse of prescription drugs within the past three (3) years; or the use of any other illicit drug (cocaine, methampetamine, LSD, etc.) within the past five (5) years. All illicit drug involvement is subject to review to determine suitability for employment.
As you can see above, the drug use you described would not automatically disqualify you with the above department five years from now.
If your record was expunged, then it should not show up on your criminal history, but a good background investigator might still find out about it.
You are young. Five or ten years might seem like a very long time, but it isn't really. Some police officers don't start their law enforcement officer careers until they are in their forties. I suggest attending college, building a good work history by maybe working part-time while you go to school. Get a four year degree. Work in another field or do security work for a while. Consider joining the military. With time and hard work, you can make yourself a better candidate.
