We like absolutes. If a person is above age B then they are mature enough to vote, or to buy and drink alcohol or whatever. If they are below age B, then they are NOT mature enough to vote, or whatever.
People know this isn't true. Such characteristics -- maturity, life lessons, family background have some correlation to age. An older individual will hopefully be more mature, and have more experience, that that SAME individual was at an earlier age. But comparing individual X at age B to individual Y at age B is entering the "apples to oranges" type of situation. Each person will develop at different rates depending on experiences, home environment, environmental (if you are growing up in a war zone you will certainly develop more quickly in certain ways -- or you will be on the casualty list). And development occurs in different aspects at different rates. One person may develop physically very quickly while their social maturity is very slow to develop. And another person may do the opposite -- they may have the physical aspects of someone small and weak but have considerable social abilities and maturity.
Another number in great use is the Body Mass Index (BMI). Yet, it is generally acknowledged by health professionals, that the number (or number range) is only accurate for about 80% of the people. A BMI is calculated by the formula of weight divided by height squared -- or kg/m(squared). But not all weight is equal. A bodybuilder with lots of muscle will weigh more -- be more dense -- at equal size. A bodybuilder, or athlete with a lot of muscle, will have a higher BMI. The same is true for people with "big bones" (used a lot as an excuse but still can be true and depending on genetic heritage). The BMI calculation calls upon a certain composition of the body -- so much muscle, so much fat, so much bone. If any of these are not in the "expected" range, the BMI number will differ and not easily able to be used for any charts or evaluations.
So, if people know it isn't true, why do we use these numbers? They are simple. Some people in the world don't know their precise birthdate but most people (especially in areas of the world where they have heard of, and use, BMI) know what their chronological age is. They have access to a scale to weigh themselves and a ruler to measure their height. It is simple -- and simple means fast and easy.
Could it be done more accurately? Sure. For aspects associated with age, a team (I would never recommend just ONE evaluation) can evaluate various development criteria. A person may come out of such an evaluation with a range of "ages" -- some higher than their chronological age and other aspects lower than their chronological age. Which is cheaper and faster -- a battery of tests by experienced professionals -- or looking at a calendar? Not hard to guess, is it?
The BMI is an attempt to give a fast and easy determination of body fat composition. But there are other ways to determine that much better than the BMI formula. One method is called a displacement test. Put someone into a tub of water and measure how much the water rises on the edges of the container. Calculate the volume displaced which is the volume of the person. Then use this volume along with the weight to determine density. Density can then be used to determine composition. A person with high body fat will have lower density. A person with "big bones" or a lot of muscles will have higher density. There are other types of tests which vary in accuracy depending on the experience and ability of the tester -- body calipers and so forth. The end aspect is that all they require extra time and extra experience and training. Some are less convenient and require special equipment. But they are much more accurate. Which is cheaper and faster?
This would be a great big "who cares" if it wasn't for the fact that these numbers are integrated into laws, and insurance actuary tables, and health advisory brochures, and so forth. The numbers can still be useful -- but ONLY if their limitations are recognized and acknowledged. A bodybuilder that is given higher insurance premiums based on being "too fat" according to their BMI should be given the option of paying for a more accurate test (probably saving themselves a lot over the term of the insurance policy).
And, when you read a news article, it is useful and much more fair to recognize the limitations of numbers. That person of "mature" age may truly be more fairly evaluated as a child. Everyone is different but it is easier to categorize them according to the numbers.
Do you know other numbers that do not always work well but are used because they are easier?
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