I don't particularly care for a time frame, I do futurity but I also don't push for it if the horse isn't ready. I agree that every horse is an individual.
However, I do start everything at two, bottom line. Even if they're the slowest, most mentally immature creatures in the world - Even if I have to spend the first month just getting them to figure out how not to trip over their own feet - And even if they're the rankest, meanest colt in the bunch.
As far as I'm concerned, the sooner they start learning, the easier life they will have. If I was killed in a car accident tomorrow, I would hate to have my family suddenly have to sell a bunch of unstarted two year olds. The more work I have into them, the better their chances of having a good life if something happened, or after they are sold.
If I have to wait four years to ride everything, then it's going to take me another year of riding to get them to their peak, and another year at least after that to get them started in a career. After that, you have another year of seasoning. So I have a seven year old horse who is still incredibly inexperienced, and the value decreases. When the value of a horse decreases, not only am I losing money, but I'm also increasing the risk that someone comes along to buy the horse who is looking for a quick turnover, or isn't a good home, or will not be able to feed the horse after a year. People who have the money to have good horses, don't buy them cheap. If someone can afford a valuable animal, usually they have the money to take care of it.
So if I start my horse at two, ride it for a year, it's already a solid citizen by the time it's three. So at three, you can start turning them towards a career, or seasoning them. So instead of a finished seven year old, you have a finished five year old. The value of that horse goes up. His chances of a happy life dramatically increase.
I want to be a part of my horses life from the time they are born so that starting them is not an issue. When they are babies, I will hold them, rub them, drag them out on the trail with me or tie them up at shows. I do everything I can to make my horses as valuable and seasoned as possible, because to me, that is how you keep horses from being neglected, abused, or sold to the wrong hands. By making them the best you can.
I keep all my flunks though. I have a horse who I loathe in my barn right now and have put thousands of hours of work into. She's a flunk in literally every way. Can't run barrels for anything. Can't cut a cow. Doesn't have any stop in her, and if you ask her to turnaround she'd just as soon throw herself on the ground in tantrum. Can't rope on her either, she's too small and regardless, she's terrified of the rope. I started three of her sisters and they all went on to be somebody's horse, all really good hearted creatures. She, however, is not one of them. I couldn't sell her because I have this feeling somebody would dump her at the nearest auction yard after spending a month with her. Unfortunate reality, but still, three years into riding her every day I am still trying to make her a horse I'm proud of. It's just not happening, and her value goes down every day she ages.
I will probably have her until she dies at this point, but I knew this early on in her career so I was able to accommodate that, and I can take my time making her into something. Maybe someday her "person" will come for her, but I haven't found one yet. The good news is, now she's so broke at six, I can turn her out all winter and she'll be the same when I get her out in spring - Because I've done my job and prepared her to be that horse, even if she was incredibly difficult. If I had "Turned her out to mature" at two instead of starting her, she'd be half as far along, and if she finally succeeded in killing me she'd most definitely be dog food.
That's just me though. I go out every day to do my job and at least take one baby step towards getting that horse broke. I don't believe in wasting time, but I don't believe in rushing either.