I might be repeating some of the things said here, but I feel the need to contribute...
- How old do drafts need to be before being backed and saddle broke?
All horses mature at approximately the same rate. Smaller horses might reach their full height or appear mature at a younger age, but the rate of growth plate closure is the same across all breeds. As a general rule, backing and light riding can begin at 4 years. You will not cripple your horse is if you back your horse at 3 but the more time they are allowed to mature the better. High impact stuff, such as jumping and galloping should wait until the horse is about 8 years old. Here is a great article for reference: http://www.equinestudies.org/ranger_..._2008_pdf1.pdf
- How old before they can be seriously ridden, competed on, etc.?
You can start competing from weanling age... seriously. Take your young horse to in-hand shows. Ride your 5 year old in walk/trot classes etc. This will get your horse mentally prepared for whatever "serious" competition you plan to participate in later. You have to start somewhere. The climb to the top happens one step at a time.
- Would a light boned draft be suitable for jumping?
Yes. Pure-bred drafts tend to be heavy on the forehand and clumsy over fences. They tend to be slower and less maneuverable than a lighter riding horse, but it is possible to find a full draft that can do a bit of jumping.
Draft crosses are commonly used for jumping. Irish Sport horses are often just a cross between and Irish Draft and a Thoroughbred and these horses are seen competing in top levels of competition. Draft crosses are often the "breed" of choice for fox hunting and eventing. The draft cross often inherits the bone, strength, and temperament of the draft parent, with some of the refinement, athleticism and maneuverability of the lighter parent.
I own a draft cross, and I jump her. I can't tell you how many times I have had people (mostly on the internet) tell me that I am crazy to jump a draft cross, that a horse that big will surely break a leg landing off a jump or develop crippling arthritis by age 10. This is all a load of crap. If you condition your horse properly, and work with a qualified professional, there is no reason a horse of that size cannot jump.
Have you seen some of the top show jumping competitors? My former boss owned a retired grand prix level jumper. She's an Oldenburg mare and she is bigger than my mare. She is 17.2 hands and built like a tank. She made her career jumping 6ft fences WITHOUT INJURY and retired to produce some equally as talented offspring. Many of the warmblood mares that moved through that facility for breeding were BIG. They were upwards of 17 hands and very well-muscled. Since they were brood mares, they carried some excess weight around the middle as well. It's no doubt they weighed more than my mare, and most had finer-boned legs.
If your horse is proportionate and has bone to match their mass (as in, not grossly overweight or big-bodied on pencil legs) and does not have conformational faults that might lead to injury (long cannons, upright pasterns etc.) and you go about conditioning and working your horse up the levels properly, your horse should be fine, even if your horse is big.
- I am considering buying a draft foal/having one bred for me. Would you go with a full draft, or half?
For your riding goals, I would go for the half draft. You will need some of that lightness and athleticism. I would also go the route of choosing a horse that is already born. Crossing two very different breeds can be a crap shoot. I have seen many lovely draft crosses. My own mare is the lucky result of a mixed breeding. But I have also seen some draft crosses with some serious faults, such as disproportionate legs or heads. The cross can also produce some undesirable safety hazards. My horse is huge, and she inherited some thoroughbred flightiness. This is a lot of horse to control when she's feeling hot. There's a reason draft horses have been bred to be docile! If you look at horses already alive, you will know what kind of conformation and temperament the horse has before buying.
- Do you put your drafts on joint supplements?
I have mine on MSM to manage inflammation of an old hip injury. If not for that, I would not give her a joint supplement. You should, however, read up on the symptoms of EPSM and the diet that is beneficial for draft horses, since they are prone to the disease: Draft horse and mule health care
- How do you find their temperaments?
It varies. My mare is shy, and skittish, but super sweet and eager to please. My sister owns a drafty cob gelding who is lazy but opinionated under saddle, and food aggressive in the paddock. My mom's gypsy gelding is cheerful, friendly and energetic. The belgian cross I rode one summer was laid back and easy going (stereotypical draft). It really depends on the individual, but overall they are more level headed and easy going than the "hot" breeds.
- When did you find yours at? I've noticed it seems easier to pick one up at auction, than from a private buyer or breeding farm...Was this the case for you?
If you have specific riding goals in mind, you should stay away from auctions. The horses that go through auctions are often lower quality, or have not had the best treatment. Even if you find a good horse at auction, it is tough to gather honest information about the horse's history, training and breeding. Stick to private sellers. I bought by draft horse from a private seller. I was able to try the horse out, ask lots of questions and get to know the seller, have the horse vet checked, and think my decision over before committing to a purchase. An auction setting does not allow you to do this. I suggest you look at sites like dreamhorse.com, equine.com, and equinenow.com and bring your riding instructor with you when you try out any potential new horses.