I've recently seen three for-sale ads were the horses are two years old and already w/t/c under saddle, a Thoroughbred, Hanoverian, and Quarter Horse. Now, I've always read that horses should start light work at three and the training  will steadily progress as they become older. What age do you think a horse should be started? I plan (in the future) to start my horses lightly at three. I think two is just too young.

What's your opinion?

In general, I think that the later a horse is started, the better off they will be both physically and mentally.  That doesn't mean you can't work on ground manners at a young age, but heavy physical work should wait until they are grown. 

Personally, I prefer to see a horse saddled and MAYBE backed once as a three year old, then not touched again until they are four with light saddle work and more extensive work in their 5th year.  But that is too much time and money to devote to a horse for most trainers, so I would not necessarily turn my back on a horse that is started with light work at 3 if they are of the faster-growing breeds.  I wouldn't even bother looking at a horse for sale that was backed as a 1 or 2 year old.  But that is just me. 

Personally, I lightly start them at three, just walk/trot, mainly trail riding. I will put a saddle on them in their two year old year, and SOMETIMES that fall I will sit on them, but not ride. And that's not always, just I've done it before when I was working on ground work/ground driving.

Calista is 4 now and soon I will just start cantering her under saddle :)

I started Nova last year as a 3 yr old. In late spring/summer he went to some lessons with me and I worked a lot at the walk and trot and got some steering and brakes installed (mostly). He had part of the summer off as I focused on our other gelding, but in September I started with him again. He was put into steady work from Sept. - Nov. By steady, I mean usually 1-2 lessons a week, with probably 1-2 other 'trail' rides during the week. Most of our riding is road riding so he got exposed to a lot of things - trash cans, cars, bikes, dogs running out, people, etc. During that period in fall we also introduced the canter to him - which he took to easily. I just want to say that while our lessons were about an hour long, we weren't working for the full hour, and he always had some breaks where he got to hang out (and fall asleep almost) while someone else was working.

Nova then got all winter off (Nov. - April) with just two short rides during that time. The weather this year hasn't been very nice, so he hasn't been worked as steadily as I would have liked. But, when I got on him again for our first lesson, he was exactly where I had left him last year. We have been making steady progress in each lesson. Last week I took him to the mountains and we did a week of trail riding there. He was awesome! We didn't put in killer days, but he probably used muscles he didn't know he had! He had to walk through puddles, through mud, across creeks and rivers, climb hills, go down some relatively steep hills, watch where he put his feet (both up and down), and just generally be ridden for 4+ hours every day.

I think that this particular senario worked out well for Nova. Other horses it might not, but he felt really mature underneath me (both mentally and physically) and he has taken everything in stride. Of course, he's a pretty easy-going guy...

ETA: Whoops, didn't answer the question really! I wouldn't start a horse before 3, and would wait until 4 if they didn't seem ready.

We definitely take the slow approach, nothing much except light lungeing occasionally before they are three, then the summer after they turn three more consistent lungeing and eventual backing after a few months, depending on their progress.  Then mostly just hauling them to different places and hacking around, maybe light collection work and some small jumping and free jumping really until they are 5.  Then, depending on what they have taken to, we begin consistent, 5x a week work.  I hate to see babies started before they are 3, they just aren't physically or mentally ready.  I would never look at a horse with any in-the-saddle time before age 3.

 I just won't buy a horse that has been started (i.e. ridden) before 3 years of age.  Horses are too good at hurting themselves and I don't believe that a two year old's body is ready for a rider.  And I think the results speak for themselves.  

 My 3 year old had 30 days put on her as a late 2 year old. Basically, She was ponied around the trails, would wear a bit, saddle, etc. She learned to stand for clipping and she would longe line all over the place. She had her ground manners down and was free longing well in the round pen. The trainer she went to sat on her twice, then she came home Shortly after, I bought her and she came to my barn and had the winter off. She has recently as a 3 year old started coming back into work. I work her 2/3 days a week. Usually we have one ground manners/grooming day where I mess with her and just have her out in the cross ties for a good 30-45 minutes. The next day we do a bit of lunging/longe lining around our barn and the next session I ride her at a w/t around a circle for maybe 15 minutes and then she is done. Some days we don't ride and some days she's behaving so well she gets two rides in a row. In november she will get the winter off and come back to work the following April for a little more work under saddle. I have cantered her once, on accident, but she lives a fairly easy life. If it were up to her she'd come out 7 days a week. She's always been extremly balanced and has taken to everything pretty well. She actually goes around like a hunter and has a great natural head carriage. Some younger horses take longer. I've seen one horse who wasn't really ready to start under saddle work til they were well into their forth year. Every horse is different, just as every person will have a different opinion as to when to start horses. I knew a lady who's three year old w/t/c and popped over crossrailsLets just say the pony was burnt out by the time she turned four and had to have a year off just to be a horse.