You need to be 21 to drive a minibus.
To drive a minibus on a car licence, under a Section 19 Permit, one of the conditions you must meet is to be over 21. You must also have been driving for more than two years.
To apply for a D1 licence to drive a minibus over 3.5 tonnes, the minimum age requirement is 21.
Section 19 Permits are issued by the DVSA to organisations that operate vehicles without a view to profit to transport their members, or people whom the organisation exists to help, such as school pupils. Section 19 permit vehicles cannot be used to carry members of the general public. So, if you are a teacher or member of staff you need to be 21 or over to drive any size vehicle, including an 8 passenger car, that some might consider a minibus.