You are as old, as your joints are (old Chinese proverb)
Flexibility is ability to bending easily; not stiff; bending without breaking. Flexibility is also a synonym for joint range of motion. Simply put, it is an ability to flex.
It is one of the five most important human bio-motor (physical) abilities (strength, speed, endurance, coordination, flexibility), defining the freedom of joint movement
So the lack of flexibility could lead to reduction of freedom of joints and of the whole body movement.
(Dr. Nicholas Romanov’s training essays: a new paradigm of training. Af Nicholas S. Romanov)