Not until they are three year olds................period.

In my honest opinion when a horse is started depends entirely on the horse. I've grown up with race horses and they are many times started as late yearlings. They get use to weight, tack, and some steering. Why? Because they want to race them as 2 yr olds and the sooner they've had someone on their back the better. Is this always the right way? No. Many yearlings can handle 100 to 120lbs on their back for a very short unstressful time (5-10min of walk). But only if they are mature enough physically. Some aren't and should wait until the spring of their 2yr old year. What breed your horse is, is very important, some breeds are just naturally slow maturers. I think most horses should be started as 2 yr olds, even if you only put in a light 30 days and then give them a few months to grow up and then restart them again. However, this still depends on the individual horse. 

Reasoning is quite simple.
Horses do not become teenagers until the age of seven, and adults at the age of twelve.

So at two, you are breaking down a baby.

I can definitely see that you have large horses, judging by your picture!
It's a really interesting topic, isn't it? Personally, I think that there is pressure to push a young horse too fast in the equine world.... I know with Morgan showing they have a Futurity program where the horses are expected to do a driving class at two and a riding w/t/c class at four. This seems unrealistic to me.

I always find this topic to be interesting as well, when I took Growth & Development there were so many ages and theories as to when to back a horse. I break it down simply as this: breed, individual horse and what are your future goals.

Prospect, I find it interesting that you breed your mares before you start them undersaddle.

I've always been thought that you can start a horse with 3 years of age. But recently I read a very interesting article about bone structure and how the dorsal vertebra is ready to carry weight when the horse is 5 years old.
My youngest horse is now 3,5 but I will wait until he is 5 before I start him in riding.
To me it doesn't matter because all animals stay here until they die so I have lots of time ahead. and in the mean time there are so many fun things you can do together.

I start them as three year olds, just basic longeing, long-lining, and sitting on them.  That's done in the fall of their 3 y.o. year, and then they're turned out in pastures for the winter in small herds. 

As young four year olds, I ride them more, but I intersperse the rides with longeing, so that they work a little every day with behavioural goals, but I don't sit on them every day.  I do start to ride them out on little hacks, and work on seeing and doing new things.

As five year olds, usually in January of their five year old year, the work starts, but I have only Training Level goals for them, and they continue always to be turned out during the days in large pastures with other horses.  If the horse seems relaxed and confident in its work, and comfortable and balanced under the rider, I might start competitions at Training Level 1 in early summer, and perhaps finish the summer at Training 2 - 4.  The horse, however, makes those decisions clear.

As six year olds I ride them 4 - 5 days per week, usually arena work, sometimes hacks, and I start to expect real physical work.  By then they've developed stamina, strength, and discipline, and the transition to work is usually very easy for them. 

Some horses develop faster, and some slower, and I think you have to let the horse guide you.  My little FEI mare has come along very quickly, and won medals every step of the way since I started her in competition as a 4 year old, but her 1/2 sister has been much slower - she's just a different horse, with different abilities.  My 5 y.o. Trakehner is coming very slowly, but she's the 3rd in this line that I've started, and they mature very slowly physically, so I'm content to take my time...