The minimum age for this licence is 18 years old and you must hold a full Cat D1 and Provisional Cat D1+E licence. There are specific rules for under 21s allowing them to drive. Please refer to the DVLA website.
Having passed the D1 tests you can now undertake the practical driver training in the D1+E category. Train to the DSA test standard and take the test.
On passing this test: you are licensed to drive a vehicle with between 9 and 16 passenger seats. You can tow a trailer over 750kg however the combination weight must not exceed 12t and the trailer must be within the rated towing capacity of the towing vehicle.