Flexible permissions for mobile sign-ins

We would love to be able to allow mobile sign-ins ONLY for volunteers 21 and older. We've never had an issue with adults abusing the mobile option, but we have with minors. We currently have no location required because we have large groups who serve and all need to check in at once. We were having lots of issues with them figuring out how to share location permissions when we had a location requirement in place. It'd be great to still offer this to our large adult groups but require minors to sign-in in person.