Yep, the law is that children must be aged 13 to ride in a tractor, on a quad, or in an RTV such as a Kubota or John Deere Gaitor. (she says desperately hoping no one saw her two year old on a combine today) Crazily, they can then drive them themselves, off road. A 16 year old can drive a tractor on the road, but they do have to pass a DVLA test to do so. The thing that really gets me is that if they pass, they can then tow a rather large trailer without having to do a trailer test.
I recently went on a free Health and Safety Executive Farm Safety Awareness day, mainly because I thought it would reduce our likelihood of getting an inspection, but I would have to say I was very pleasantly surpirsed at how interesting it was, I would definitely recommend it if you get the chance to go.