When young, things are soft.
When old, things are brittle.
He goes on to say, "Metaphorical stretching leads to expansion and flexibility in personal growth. A young plant is tender and pliant. An older one is stiff, woody and vulnerable to breaking. Softness is thus equated with life, hardness with death. The more flexible you are, the greater your mental and physical health."
For me stretching has taken the form of hatha yoga for the past thirteen or so years. Flexiblity in other areas of life takes daily practice, too.
Some days a yoga posture is easy and strong, on other days it is not so. It depends on the time of day - morning yoga is wonderful but your flexibility in not as great as the end of the day. I prefer evening yoga after taking a hot shower and just before going to bed. As well, it is best for me to consider matters of importance after daily rising, drinking two cups of English Breakfast Tea and reading a bit of Joseph Campbell. You must discover when your body, mind and spirit are at its best for "stretching".
Lean flexibility is synonomous with youth; however, metaphorical stretching comes with experience and age.