MyBB Community Forums

I want to only allow people 21+ on my site. Is there a setting for that? Or a plugin?

A website cannot be restricted by age.
Either your website is online or not!
If you're running a MyBB forum, you can create user groups and put members to groups manually, when these proved their age.
Forums/subforums can be hidden for guests, but free for members of a specific group you granted.

[ExiTuS]

I don't want to block users or make a group. Just need something that says you're 21+ when registering. I have the forum closed to people that aren't signed in.

Should I just put it in the "I Agree" section in registration? Where do I edit that text?

Yes, it's possible to put a note to the agreemant page before registering.

Go to ACP > Configuration and edit the language pack:
member.lang.php > aggrement*

[ExiTuS]

Thanks. I'll just add something there. Thanks!