One thing to be aware of: nothing in juvenile court automatically drops off your record at age 18. You generally have to apply to the court where your case was heard in order to get the record sealed.
As far as Canada, if the charge was dismissed, you should likely be able to enter. However, you will never know for sure until you get to the border and request to enter. The border officials have a great deal of power and authority in deciding who gets to enter their country. If they decide you cannot enter, then you have to turn around and go home. You may want to consult with an immigration attorney who is familiar with Canadian law to get more detailed advice on your situation.
I agree with the other answer, but would like to add a little. If the charge was actually dismissed, then you might be able to enter Canada. You will need proper travel documents, and you will need to determine that your charge was actually dismissed, and that Canada will let you in. I would check before I got to the border, but that is up to you. You can find out more about their requirements here: