I've been towing trailers since I was 16, and towing my horse in them from about 6 months after that. Here in NZ, you can tow a trailer at any license stage, provided you have your guardian with a full license in the passenger seat. As soon as I got my restricted license I was legally able to tow my horse on the roads.
Now that I'm 18 and have a full license (albeit very newly) I tow my horse everywhere. The last show I took him too was an hour and a halfs drive out there, into no cellphone reception, up and down huge hills with tight corners... I'll be honest, I was scared out of my mind until I got through the first set of hills and tight turns, but after that it was ok.
It really does depend on the law where you are though, and also what type of insurance you have, if any. Everything I drive that counts as a tow vehicle, is always fully insured, and so is the trailer I tow with, even though insurance isn't compulsory here.
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms