The favorite word in almost any child's vocabulary is the all-inquisitive "why?" Why is the sky blue? Why do I have to eat broccoli? Why can't I stay up late? Why? Why? Why? All of these questions present varying degrees of difficulty based on your knowledge-base or your authority. I would like to deal specifically with questions wondering why you do/don't want your child to do something.
Most kids question their parents' authority and/or motives or reasoning behind parental decisions such as bedtime, game privileges, clothes, etc. The parental dilemma revolves around what kind of answer (if any) the child's question requires. Should the child receive a detailed explanation as to your logical reasoning behind selecting 8 p.m. as their bedtime? Or should the child simply be reminded of your authority by hearing "Because I said so"? Someone once told me about a book or article that discusses explanations for kids based on their age. Unfortunately I don't remember all of it exactly, but I agree with the basic principles I do remember.
Ages 0-6 years: At this stage in a child's life, discipline without question is highly necessary. These are some of the most formative years, and the respect for a parent's decision regardless of the child's feelings about said decision is crucial to a successful raising of the child. Explanation behind why the child is to stop hitting their brother or to go to bed now is not necessary. Rather, it often detracts from the commanding emphasis of tone. Furthermore, trying to explain to a four-year-old child why they can't stay up until all hours is both frustrating and futile. Therefore, at this age, keep to a simple "because I said so" and save yourself a lot of bubbling tempers.
Ages 7-12 years: 7 is the age of reason (although arguably it begins much earlier). Regardless, 7 is the age when kids become morally responsible for their actions and any consequences. (This does not mean that discipline and punishment before age 7 is unnecessary. Rather, it is more so because an obedient character should be firmly in place as the child grows older.) At this stage in a child's life, they have a better understanding of right and wrong, dos and don'ts, yes and no. This doesn't meant that they are necessarily entitled to a descriptive explanation behind your decisions, but they can be trusted with a brief statement outlining your actions. As your child grows older, you can treat them more as an adult (although most definitely still a child). As they mature, giving them brief explanations oftentimes helps them understand further the reasoning behind obeying your word. They should still obey you pleasantly, but they can validly question why they have to go to bed so early and expect to receive a short reason rather than simply "because I said so."
Ages 13-18 years: Finally, as your children enter into their teenage years, the more difficult questions begin. Why can't I go out with him? Why can't I see that movie? Why can't I drive out there? At this point in their lives, your children should have a good mind developed due to your training and their schooling. This means that they will be able to understand your reasoning, if they apply their own rationality to the answers. At this age, a more detailed explanation behind your reasoning often grants more weight to your decision than a simple "because I said so." The teenage years are often the most rebellious, and your child is apt to be more prone towards flagrant disobedience if all they hear is "No" and "because I said so" in answer to their ever more frequent questions. They are young adults and deserve the proper response due to them. While still respecting your authority, you can discuss your decisions regarding your teenager's privileges etc. If you treat them like an adult and NOT as a little child, you will get a far better response than the ranting and raving that often ensues between the strong-willed parent and child. If you treat them as an adult, they are more likely to respond as an adult rather than a complaining child.
Beyond the age of 18, your child is an adult. They are no longer legally bound to do as you say, although one would hope that they would still respect your authority and pay heed to your sage advice. Furthermore, if they continue to abide under your roof, you have every right to set some ground rules for them to follow. You cannot necessarily dictate what they do and who they see, but you can still maintain control of what goes on within your own home. Regardless of where your child is at the precipice of adulthood, keep in mind that (hopefully) you have done all you can to raise your child properly. It is now up to them to act on or disregard the sound moral principles you have ingrained within their character. Please, continue to offer advice and counsel, but don't expect your child to always heed your words or take everything you say to heart. Remember that they have their own minds and ideas about how to live their lives. Oftentimes, their ideas are just as good as yours, simply different. Let them make their own mistakes, but be there when they ask for help. Believe me, your child will realize (sooner or later) how much they cherish your wisdom and comfort.
Therefore, the answer to the question "Why" varies depending on the child, their age, and the pertinence of the actual question. The answer of "because I said so" can be used at any age and should be followed without question based on your authority and your child's obedience. Further explanation concerning your parental decisions can be introduced as you see fit, preferably once the child is a bit older. These explanations will, most likely, become more frequent once your child hits their adolescent years. Remember, maintaining a good relationship with your child is crucial. They must respect your authority, but at the same time, you should treat them according to their age. Don't talk to your 16 year-old like she was 3 years old. And don't explain to your 5 year-old like she was 17 years old. Keep in mind balance and moderation. And good luck!