Hi Dudley6623,
If you have a full UK car licence (and hence you will be over 17), you are fully legal for any normal tractor and trailer without having to take an additional tractor test.
Things like tracked machines (Crawlers) will need a track layers licence, whereas larger machines like combines you should be over 21 to drive on the road if they weigh over 7.5 tonnes. If they're between 3.5 tonnes and 7.5 tonnes then you need to be over 18.
Hope this helps, but have a look at the DSA website for more information.
Emily