You can create a 21+ age limit on ticket levels. To do this, use the toggle button below the ticket level in Step 2 of the Wizard:Next, set up Per-Ticket Information in Step 3 of the Wizard (Purchase). Select the ticket group and then Personal Information Collection>>Per-Ticket Information. Make sure that Age is required. If it is not, the 21+ verification will not display.
When enabled, the personal information collection page will show an error if someone under 21 tries to buy a ticket in a 21+ level: