Flexibility is defined the "absolute range of movement in a joint(s) that is attainable in a momentary effort". What does this mean? It’s the total amount of motion a specific joint can move through. For example, the knee can have 150 degrees of total motion. They key point is specificity. Flexibility in one joint does not equal flexibility in another or all joints. Having a shoulder joint that has full motion and good flexibility does not mean the knee joint in the same person will be automatically flexible. flexibility can also be directional in a joint. A hip can have good flexibility in flexion and extension/ “forwards and back" but limited in abduction and adduction/“in and out”. It's why someone can do left or right splits but maybe not center splits.
There are some conditions and factors that can effect a persons flexibility. Increasing age, activity level, and medical conditions can alter a person’s flexibility. However, these are factors that influence flexibility not determine it. Flexibility can improve for anyone but patience and determination are required