How old is this person?

Typically, x-rays are made by passing electromagnetic radiation through an object. The rays which pass through the object easily, hit the x-ray film and create a black spot. If the rays are blocked by a particularly dense item, the rays can't penetrate the object and reach the film, therefore a white "shadow" or picture is created. A carefully calibrated x-ray machine can create a picture that shows the images that appear on a coin that has been x-rayed. It can do this because various parts of the coin are different thicknesses and block various amounts of radiation. There are many lovely works of art that have been created by performing x-rays of shells and other objects of varying densities.

Luckily for us, bones are dense enough to show up on x-rays. In fact, if you are a female of my age, I hope you've had a DEXA scan or similar test to check and see if your bones are sufficiently dense to avoid fractures caused by osteoporosis. Link

Cartilage is not dense enough to show up clearly on most x-rays. Cartilage is the bendy stuff that is found in the tip of your nose and the pinnas of your ears (the part of the ear that sticks out on the sides of your head). Cartilage is also found on the ends of your bones where it creates a cushion. The next time you eat a chicken or turkey drumstick, check out the bone that is left. See that greyish shiny stuff on the end? That's cartilage. (Certain types of arthritis are the result of cartilage being worn away from the ends of the bones resulting in painful friction.)

In a developing baby, the bones of the skeleton start out as cartilage. As bones grow and mature, calcium is deposited and the density of bone increases, causing them to get longer, larger in circumference, and harder. If you look at an x-ray of a newborn baby, there are large gaps between bones. These gaps contain the cartilage that will continue developing into bone. Of course this cartilage is not as flexible as that in your nose and ears, but it is cartilage nonetheless.

If you look carefully at the x-ray of a child's hand bones shown here, you can see the spaces called epiphyseal gaps (some of them are labeled). These gaps are filled with cartilage and this is where bones grow in length. Once a person reaches adulthood and stops growing, these gaps fill in with solid bone and no longer appear in x-rays.

If a doctor x-rays the hand and wrist of a newborn infant, the carpal bones do not appear because they have not yet begun to calcify (deposit calcium and harden into bone). They're still made of cartilage, which doesn't show up on x-ray. By the time a child is one year old, one of the eight carpal bones begins to calcify so one wrist bone would appear on an x-ray. By the time a child is two years old, a second bone begins to harden and shows up on film. During the third year of life, the third bone forms, and so on until all eight carpal bones have formed. If a physician has a concern about a child's development, an x-ray of the hand and wrist can offer a clue to tell the doctor if the child's bones are growing at the expected pace of one carpal bone per year. How many carpal bones does a nine-year-old have? Eight. The maximum number is eight.

Take a close look at the x-ray shown. Link How old is this child?