Motor Vehicles (Driving Licences) Regulations 1999
Section 9 (3) states the minimum age for driving a tractor is 16, where the tractor is less than 2.45 metres wide, and “is driven either—
(i) without a trailer attached to it, or
(ii) with a trailer which has an overall width not exceeding 2.45 metres and is either a two-wheeled or close-coupled four-wheeled trailer,â€. Otherwise, the minimum age is 17.
Schedule 2: Identifies tractors as category F licences and mowing machines or pedestrian-controlled vehicles as category K licences.
“A mowing machine is a specialist ride-on grass-cutting vehicle with permanent cutting equipment. You must be 16 to use this type of vehicle.â€
(https://www.gov.uk/learning-to-drive-a-tractor-or-specialist-vehicle/rules-for-test-vehicles