Do i need a US passport to visit Canada?

If you're American, you don't need the full passport but you will need a passport card. You can get either through the Passport office web site.

you need a passport to go anywhere outside of the US

Effective June 1, 2009, the US Department of Homeland Security set new rules in place for people travelling to the US. This applied to all individuals crossing the border into the US, including US citizens. Effective June 1, 2009 the only acceptable documents for cross border travel are: A valid passport - Valid for all forms of travel A valid passport card - Valid for travel by land or sea An enhanced drivers license (or state ID) from those states offering them - Valid for travel by land or sea A Fast Card such as a Nexus Card - Valid for all forms of travel A "Green Card" - Valid for travel by land or sea A youth, 15 years of age and younger can travel by land or sea with an official birth certificate. First Nations of Canada can use their government issued Certificate of Indian Status Card for travel by land or sea. Since any person crossing into Canada must be able to prove their ability to return to the US, the same rules apply to a US Citizen crossing into Canada.

As a US citizen, there are very few places where you can go now without requiring a passport or other valid document. This is because the Department of Homeland Security has tightened the rules for identification of travellers. Just do a search on the Western Hemisphere Travel Initiative, or WHTI for short. It used to be that Canada and USA would allow their citizens to come and go with just a "verbal" confirmation. In other words, you would be asked "What is your nationality?" and you could just answer "American", and after a few other questions, Canada Customs would let you in the country. Now, because the US Border Protection demands that all citizens present valid travel documents, American citizens actually need to carry proof to get back into USA. That's what the passport is for. There are also other suitable alternatives (Enhanced drivers license, green card, military ID, NEXUS card, etc) but the passport is probably the easiest and cheapest to get. It is also a travel document that is the most widely accepted in all countries (since Enhanced drivers licenses are only good for Canada and Mexico). I think the only places you are can fly without a passport now is on domestic flights within the continental USA, as well as flights to Alaska, Hawaii, and Puerto Rico. Any flight that is considered "international" will likely have the restriction of needing a valid travel document to board.

if your older than 15 you need a passport

What makes Americans think that they are so important that they won't need a passport to enter Canada, but Canadians need a passport to enter the States? Yes, you need one.

You do not need a passport to enter Canada, but you do need one to get back into the USA. This is an American requirement, not a Canadian one. Canadians hate it!