Had another thought that might help the understanding:
I would relate horses' physical maturation to the human maturation process as follows:
A two year-old horse is like a 12-14 year-old person. Just past puberty, getting strong, but still growing. Musculature and coordination are still a bit out of sync, because the skeletal structure is growing so quickly. Able to do hard work for short lengths of time, but be careful. Willing, but lacks the mentality for serious work for more than short time spans.
A three year-old is like a 15-18 year-old, whose skeletal structure is now sound and just about as physically sound as it will get. Coordination is getting good now, but there is a lot of muscle yet to be built. Still has a tough time staying focused on the task at hand. Likes to test the limits, very impressionable. Here is where training and discipline can make or break both horses and humans.
A four year-old is like a 19-22 year-old person, who is now college-age and is fully capable of handling anything a human should be able to handle, as far as skeletal structure is concerned. Now the task is to build muscle, continue to train coordination, but mostly to train the mind.
A five year-old is like a mid-twenties person, who is now fully capable, both physically and mentally, of being a contributing member of society. Physically capable of doing whatever the gifts/limitations God gave them will allow. Mentally, they should now be able to focus on whatever task is at hand for whatever amount of time is required. Of course, in both horses and humans, their individual personality, prior training, discipline, and circumstances will have a lot to do with their physical/mental/emotional health and maturity, and personal happiness, from here on out.
One thing I know: Proper discipline and training at all stages makes happy people and happy horses (ergo happy horse owners).