As a self employed sole trader, he could be any age (but it would be rather difficult to take funding). To be a director of a corporation, he'd need to be 16 now (although prior to 2006 it was any age, with provisos).

British people become relatively emancipated at 16. You can leave school (I did!), go to work, join the army, smoke, have sex (sadly, I didn't), and do many things. Just not vote, drive, drink, and a few other things.