My way of thinking is that if I would have a dojang I would belt sistem I will use for my students would be: white, yellow,green,blue and red, the minimum time to obtain the next belt will be one full year, this means to obtain a Black Belt the student must have a minimum of 5 years as a colored belt.
As a school owner, you may do as you wish, and certainly I wouldn't fault you for it. But make sure that your reasons for extending the time to more than twice the average are for the benefit of your students and not for some kind of time in grade bragging rights.
Also, don't compare taekwondo time in grade to BJJ time in grade as others here have. Taekwondo and Brazilian jiujitsu are different arts with no real commonality aside from both having a competitive element and both having belts.
In Korea, it takes a year. Most schools that I have visited average two to three years. And in KKW taekwondo, first dan is still considered a beginning grade. Someone earlier said something along the lines of, 'I can see it for higher ranks, going from red to black...' Well, red belt isn't a high rank at all.
I don't personally care how long someone takes to get from white to black. That is between them and their instructor. The quality of their practice will show in what they do.
a child below 18 years achieve black belt status?? Yes but not a black belt to wear on his/her waist, I will use the poom blet (red/black).
Why eighteen? The KKW standard for a dan grade is fifteen. If the org has issued a dan grade, why would you issue a pum belt??
I think I will not acept children below 10 years.
A lot of schools have age minimums. You need to determine what is right for you in this regard.
To those who have asked about getting a black belt without self defense, how are you defining self defense? Learning to avoid blows and to block and strike with various parts of the body qualifies as self defense. It may not be the most thorough SD curriculum, but it is 'self defense' in the general sense. Does BJJ have 'self defense' beyond grappling? I don't practice it, so I don't know first hand, but so far as I know, BJJ is about as SD oriented as taekwondo, one having few grapples and the other having no strikes whatsoever. Haedong gumdo also has no practical SD element. Should those arts not issue a black belt?
Just to clarify, a black belt in KKW taekwondo signifies that the student has proficiency in the geub level material taught in a particular school, with the material in the KKW textbook serving as either the entire curriculum or as the foundation of the school's curriculum. Thus not all schools will be identical in this regard, but should share the same foundational skills.