Flexibility isn’t about being able to bend over and put your palms on the floor or do the splits. Flexibility is just what it says; your ability to flex, to adapt, change, respond to changes in your environment.
Mobility, stability, strength, coordination, strength and stiffness all go into making us flexible and they can all be practiced and developed. One of the keys to developing them is choosing the activities that actually help you develop them. Another factor is matching level of challenge with our ability to recover, learn, and adapt or grow.
The ability to change and respond in healthy ways to the changes in our environment (internal and external) is a good measure of the health of any organism. A much better measure than whether or not you can sit in the lotus postition.