Canada
In Canada all products with a label containing the words pepper spray, mace, etc, or otherwise originally produced for use on humans are classified as a prohibited weapon. Only law enforcement officers and individuals/corporations who have special government permits may legally carry or possess pepper spray. Any similar canister with the labels reading "dog spray" and/or "bear spray" is regulated under the Pest Control Products Act - while legal to be carried by anyone, it is against the law if its use causes 'a risk of imminent death or serious bodily harm to another person' or harming the environment and carries a penalty up to a fine of $500,000 and jail time of maximum 3 years. Of course, the legality of using spray intended for animal deterrent on a person would be determined in court on a case-by-case basis.
United States
In California, the container holding the defense spray must be less than 2.5 Oz.
In the state of Maine, criminal usage of pepper spray or similar products is a violation of law, however usage of said products for self-defence as well as possession are legal. In Massachusetts, residents may purchase defense sprays only from licensed Firearms Dealers in that state, and must hold a valid Firearms Identification Card (FID) or License to Carry Firearms (LTC).
In Michigan, pepper spray is legal if it has less than 2% of the active ingredient, this decreases the length of the effects but not the SHU. Sprays containing a mixture of CN/CS are also banned.
In the state of New York, pepper spray may be legally possessed by any person age 18 or over; however, it must be purchased in person (i.e. cannot be purchased by mail-order or internet sale) either at a pharmacy or from a licensed firearm retailer (NY Penal Law 265.20 14 (a)), and the seller must keep a record of purchases. The use of pepper spray to prevent a public official from performing his/her official duties is a class-E felony;
New Jersey allows non-felons over the age of 18 to possess a small amount of pepper spray, with no more than three quarters of an ounce of chemical substance.
In the State of Washington, persons over 18 may carry personal-protection spray devices. Persons over age 14 may carry personal-protection spray devices with their legal guardian's consent.
In Wisconsin, tear gas is not permissible. By regulation, OC products with a maximum OC concentration of 10% and weight range of oleoresin of capsicum and inert ingredients of 15-60 grams are authorized. This is 1/2 oz. and 2 oz. spray. Further, the product cannot be camouflaged, and must have a safety feature designed to prevent accidental discharge. The units may not have an effective range of over 20 feet and must have an effective range of six feet. In addition there are certain labeling and packaging requirements: must state cannot sell to anyone under 18 and the phone number of the manufacturer has to be on the label. The units must also be sold in sealed tamper-proof packages.
In many (but not all) other states, pepper spray can be purchased at various stores and carried legally by anyone over 18. However, many states do not say anything about age.