You can create a 21+ age limit on tickets if you enable Advanced Tickets (Tickets >> Advanced Tickets). To do this in your event dashboard, navigate to Tickets >> Setup, and click the Edit Icon for the ticket you would like:

Click the 21 and over checkbox, and click Save:

Next, set up Individual Ticket Information. To do this, navigate to Tickets >> Information Collection, and click the Edit Icon for the selected ticket group. Turn on the Individual Ticket Information toggle, and 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: