Sam (age 15)
I cannot claim that the calculator is absolutely perfect. First off, it depends on the accuracy of the information that you give it and not everyone is perfectly honest about themselves. But because the calculator is anonymous and objective, it does tend to give reliable answers. The second problem is that information being judged are secondary sexual characteristics. In other words because you are growing up, certain external characteristics appear. Those characteristics are not the cause of growing up, just the side-effect of maturing. Since these characteristics tend to follow a certain pattern, by measuring the secondary characteristics, we can make a good guess about the primary events that are internal.
There is a more accurate standard that a doctor can use. He takes an x-ray of your left-hand and then compares the bone shapes to a list of standards to determine your "bone age." If the growth plates in your bones are sealed off, then you stop growing. This is more direct evidence than guessing from the secondary characteristics, but the results are rarely different.
When Dr. Tanner developed his scale of development during adolescence, he defined stage 5 as the point in which growth in height stops. All development doesn't stop immediately, there are still a number of internal changes that will continue for several years -- mostly in the brain. It is possible for a 15 year old boy to have reached stage 5, but it means you reached puberty (the start of your changes) at an early age, such as eight or nine.
People have long suspected that age of initial puberty is creeping downwards. Currently that average age for a boy is 11 and a half to show the first signs of reaching puberty. Because you started earlier, it also means you started at a shorter height. Thus when you finished you were shorter as well.