How old do you have to be to ride a 50cc dirt bike in Britain?

You have to be at least 16, you also have to attatch an L plate if you do not wish to take your test.

Edit to the above answer: I've just passed my test so all this is still very much in my head. If you're off-road on private land you can ride any bike at any age. If you want to ride ANYTHING on the roads you must be at least 16, have passed a CBT (Compulsory Basic Training) course and use L-plates. You can't just stick L-plates up and ride something on the roads these days, you will be stopped and asked to show your CBT certificate. This CBT certificate lasts for two years in which time you need to either take a Theory Test and then your full practical test, or re-sit your CBT.