Potty training is often a condition for preschool, and if your child is not completely potty trained they may not be welcome in the preschool classroom. Typically, preschool starts at around age 2.5 to 3, so you should plan to have your child potty trained by then.
You will typically begin potty training your child between the ages of 2 and 3. Look for the "ready" signs. Links included.
Children develop at different paces, therefore there is not a designated set age of a child to be completely potty trained. It will vary from child to child and boys seem to take more effort and a little longer to "get it" than girls. Which is understandable.
When potty training your child, just remember the importance of praising their efforts, possibly a reward incentive and never be negative about the experience. They will get it, just keep it a positive experience.
To avoid bed wetting, limit liquid intake just prior to bedtime and encourage them to use the potty before laying down, maybe just before they brush their teeth-just as a general rule, this may help prevent the "changing of the sheets" later in the night.
Remember, age is not the most important factor, so therefore, it really doesn't matter. The child's physical and emotional development will determine when a child is ready and when they will master "the potty". By example: My daughter mastered it by 3, but when my son was born, she inverted. Soon, she adjusted to the newborn baby and resumed using the potty with no problems. My son, would urinate in the potty around 3, but did not use it for bowel movements until 4 - 4 1/2. It seems confusing for boys, sit/stand or both, but they will all get it, just some sooner than others. Stressful situations (such as a newborn baby) will alter their progress, but do not be alarmed, it works out. Stay positive.