You can create a 21+ age limit on tickets. To do this, use the checkbox beside the ticket in Event Dashboard >> Tickets >> Create or Edit:
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 21+ ticket: