You could feasibly start under 21 by entering the Young Driver scheme. Not sure of the details, but you are sponsored by a firm and have to drive for them until you are 21 (and possibly for a minimum time after that). The pros and cons vary a lot depending on what you are doing. I have been an agency driver...which is not too bad, but wasn't brilliantly paid then (lot better now) but take all these ads that say you can earn �600 a week with a pinch of salt! I have been an owner/ driver for 11 years now. Basically in this country you are treated like a piece of sh!t by most warehouses, other drivers etc. On the continent you are treated like a professional. I used to love driving in Germany and Austria (though the French cops are bast@rds!), but as there are so many East European drivers doing continental work now (let alone over here), there are very few jobs to be had. In this country there is a massive driver shortage, but you will have to get used to being in constant hold ups etc. You could put yourself through your test (you have to do a rigid first now before you are allowed to take an artic test), but it can be quite pricey. Google for HGV instruction. Otherwise write to any big transport firms near you. Many will put you through your test as long as you sign up to work for them for 12 months or something.
Personally...I love it!