Is this true that a human attains the maximum height till the age of 21 years?

Yes it's true that by the age of 21 years, by virtue of human physiology all the bones of the body gets connected and no further growth is seen. But it is incorrect to state that after this age he/she cannot grow in height; this is assumed because the growth is not visible in the legs it is normally above or below the back area.

Does this mean that after the age of 21 years height will not increase?

No, there are many cases, where people grew in height after the age of 21 years.

I am 27 years old, can I grow my height, if yes then How?

No doubt you can increase your height at this age. At times due to wrong postures and improper positioning of body parts your height gets shortened. Working regularly on correcting the body posture can help increase height. Proper intake of a balanced diet, exercise, and rest and sun bath can not only impact the height but also improve your mental health. Go through the suggestions in this book and follow the instructions in the course Improve your body postures and increase your height, and you will definitely see drastic changes in yourself.

How do we know that the spine is flexible and stretchable?

You need not prove this fact when you can yourself feel it. Starting from the neck till the end, just below the waist, is the Spinal cord. This is quite evident that your spine is flexible, as you can very easily bend, move, and turn your neck, back, and waist.

To test, you can do the following. If you measure the length of the spine when somebody is standing it would be, say, 26 inches. Now the same length, when the person is in a bending position to touch his feet, would be about 28 inches. Hence, this proves that the spinal cord is flexible and stretchable.