Also, note the terms of service from Mt. Gox:
By opening an account to use the Platform ("Account") Members represent and warrant:
they are at least 18 years of age and have the full capacity to accept these Terms and enter into a transaction resulting on the Platform
[Update: I see CA VirtEx's terms of service include the restriction:
"Your use of the VirtEx site shall serve to confirm that you are both 18 years of age or older and a Canadian citizen.".
As long as you deposit using the cash at a bank method, I don't believe there is any need to go through the account verification step. ]