Yes, so do I. It is more likely that your type will progressively change during your teenage years and into early adulthood, due to the onset of mood swings (particularly in my case), peer pressure, increasing intelligence and social awareness, greater economic-related desires (may be new, trendy clothes) and so forth. For instance, I went from a probable INFJ as a 13-year old, with strong I and F functions, to an INTJ, with strong I and J functions. Your type can also change when you are stressed or experiencing a heavy workload, as put forth by some websites. In this case, I turn into an ISTJ.
The debate, really, is whether you are actually "changing" type, or whether you are simply adapting to your surroundings by (1) developing parts of yourself that are not your "preferred" functions or (2) abandoning what were just adaptations of yourself to old stressful situations and now revealing your "true" type. This is all done in the name of survival, even if we do not recognize it.
For examples of #1, sometimes you will be forced to acquire a job that demands your skill in areas you really are bad or just average at, in order to survive, because no other job opportunities exist. Or an introvert (for example) might be tossed into a situation where they have to extrovert/interact more than they enjoy in order to survive.
For examples of #2, a child who is very different than the parents and/or siblings will (if the parents are stubborn or dysfunctional or otherwise naive about type differences) sometimes learn to play roles that do not mesh with them... they have no choice, the parents hold the power, and if they do not wear the false clothes, the parents will punish them in some way (either directly, or withholding love and/or resources, etc.) Or perhaps the child goes to a school where all their peers are generally representative of one type and they stick out badly, and they feel that -- to fit in -- they need to act like everyone else.
Especially in the formative years, then, it is hard in these sorts of situations to recognize what is the "true self" and what is the "false/studied" self.
In some ways, the differences are purely academic; but people who are "free to be themselves" usually are much happier and feel much more in sync with life. When you are acting constantly out of your non-preferred functions, it is like driving a car down the highway with a parking brake on; even if you can get the car up to speed for long periods of time, you're still "dragging" or even damaging the vehicle, and once you release the break, things suddenly become much simpler and easier and more pleasant.
This doesn't mean that you cannot become skilled that things that are not naturally "you." Even if later in life you realize "who you are," you can still use what you have learned and the non-you traits you agonized to develop in service of your "true self." So it is not a complete loss. But one really has to have a sense of how one is wired.
As far as percentages go on the MBTI test (like saying you are 60% T and 40% F), well, most of the time those numbers mean nothing -- because you're only being asked yes/no questions. If you T is a little stronger than your F (even just 51%/49%), when you answer T/F questions ("which do you prefer?") you will still pick T every time and you could come out with 100% T. These tests are not really figuring out the actual strength of your preference... just which one preferences is dominant.
The only time it seems useful is when you are very close to 50/50 or when you are extremely strong.
A different sort of test is needed to determine actual strength. (I believe this is what the Duniho test was trying to do, with asking T/F questions and then asking you to evaluate the actual strength of the preference as well.)