If you stretch regularly and correctly, stretching can benefit your weight training tremendously. Stretching the fascia allows for maximum muscle growth, and it decreases soreness resulting from weight training. Most importantly, stretching regularly can help you prevent injuries. Tai Chi is also an excellent internal martial art that helps with many conditions and improves flexibility. Flexibility Theory Flexibility is joint specific.
Some joints can be more or less flexible than average.
Flexibility in a joint depends upon the joint surface, the length and elasticity of ligaments, and the elasticity of muscles. Flexibility can be improved upon by anyone and at any age. Obviously, if you intend to extend your flexibility in dynamic movements you should use dynamic stretching. With weight training, we will use primarily static stretches and isometric stretches. I would suggest that you do your static stretches after you have warmed up on the treadmill.