You may be able to do something about the fact that the tenant sublet his unit without your consent, which would be to file an application with the Landlord and Tenant Board to terminate the tenancy and evict the unauthorized occupant. However, there is nothing you can do with regards to the person being a minor. The Residential Tenancies Act does not make any reference to age, in fact under the Human Rights Code, landlords cannot deny accommodation based on the person being under age.