Most children begin to prefer using one hand over the other for skilled activities (e.g. eating meal with a spoon, throwing a ball at a target) by the time they are in preschool (age 3-5 years). Technically, they aren't considered "behind" if they haven't done this even up until the age of 7 years when their brain becomes fully myelinated. Yet, some developmental standardized tests show that it's a skill that should be present at 2 years of age. It helps if the child has chosen a hand dominance by preschool and kindergarten when they are learning to write letters and draw pictures. Since 96% of people are right-handed, chances are you can put the crayon in the child's right hand and be correct about your choice. But definately, don't fight with the child if they start using the left hand, just let them do it. Maybe they can perform an activity such as cutting out shapes by taking turns with each hand. And eventually, one hand will feel "just right" to them, and they will choose a preference.
Not having chosen a hand preference yet should not be confused with being ambidextrous, which is being highly skilled in each hand such as writing just as neatly with either hand, manipulating chop sticks with either hand, or hitting the baseball with the bat in either hand. Often, it may not be known if a child is ambidextrous until middle elementary school years, since preschoolers are just learning to perform fine-motor activities.
So what is dominance confusion? Well, this is when the child isn't that skilled in either hand, and may only use the side of the body that is convenient for that moment. For example, if you placed a spoon to the left side of the child's plate, then he would eat with his left hand...and would probably be messy with it. If you had placed the spoon on the right side of the plate, he would have used the right hand to feed himself. He may not cross over the middle of his body as noted by not transferring items from one hand to the other and only reaching with the hand closest to the item he is about to grasp. This might also accompany other problems since crossing the midline of the body requires that the two sides of the motor hemispheres of the brain "talk" to one another. Often, I see global muscle weakness in these kids and a poor ability to perform 2-handed activities such as holding the paper with one hand while the other writes on/cuts paper or one hand holding Mr. Potato Head's (R) body while the other hand inserts body parts . This is known as poor bilateral (two sides of the body) coordination which is often seen with certain syndromes (e.g. Down), cerebral palsy, sensory processing disorder (AKA sensory integration dysfunction), prematurity, and developmental delay. Anytime you see poor bilateral body coordination, there is a tendency for the child to have a speech and language delay, since the mouth is on both sides of the body. These children might also be clumsy such as with climbing, falling when running, not jumping well, and delayed with learning to ride a bicycle.
If you are wondering if your child is ambidextrous, get a checklist of fine-motor skills a child his/her age should be able to do. Then, have the child perform all of the skills with the right hand, then later do the same with the left hand. If he could do good with both sides, then he may truly be ambidextrous.
Also, remember that a person with a right hand preference could be good at using the left hand for certain things, especially when taught a certain skill with the left hand. For example, I was taught to tie my shoelaces by my left-handed sister, so I do that skill like a left-handed person would do. I have tried to change how I tie shoes, shoot pool, and perform the other numerous skills she helped to teach me, but my brain has already learned those things using the left hand. Now, if I were ambidextrous (which I'm not), I would have been able to generalize those skills to the right hand and make the switch easily. So are you a lefty, righty, or amby?