In the case of an account requesting 'Cash' trading permissions the minimum age requirement is 18 (for joint accounts, both parties must be at least 18 years of age).

In the case of an account requesting 'Margin' trading permissions the minimum age requirement is 21 (for joint accounts, the primary account holder, at a minimum must be at least 21 years of age.  The secondary joint account owner must be at least 18 years of age).