The historical measurement results will not be updated with the full measurement results once you turn 18. However, you will see that from your 18th birthday, more measurement results become visible than just the total weight, BMI and fat percentage.
The reason that you do not get all the measurement results when you are under 18 is because the body composition of people under 18 can be very different from each other because growing up. One person develops to an adult body composition faster than another in the same age group.
The studies of the body compositions of persons under the age of 18 are very time-consuming and complicated. These studies are included in a number of our professional scales which you can find in hospitals and dietician practices, but therefore not in our consumer scales.
Read more about the technology TANITA uses in its scales here: How does TANITA measure body composition?