In the book The Science of Flexibility, author Michael J. Alter says,

"The pelvic regions of the men and women allow the female human body a greater range of flexibility. Men's pelvic bones are generally heavier and rougher; the brim is not as rounded; the cavity is less spacious; the sacrosciatic notch, pubic arch, and sacrum are narrower; and the acetabula are closer together than women's. Generally, women have broader and shallower hips than men and therefore a greater ROM (Range of Motion) in the pelvic region. In particular the shallowness of the women's pelvis permits a greater degree of joint play."

Alter goes on to say, "However, even among women, pelvic types vary, and each has its own influence on ROM."

"Flexibility can be developed at any age, given the appropriate training. However the rate of improvement will not the be same at every age, nor will the potential for improvement. Specifically the greatest improvement occurs between the ages of 7 and 11 years. By 15 years of age, the indices of mobility in the hip joint are maximal, and in later years, that amount decreases." That being said, "Evidence suggests that even senior adults benefit from exercise programs for developing ROM. Maintained or increased use of full joint range could help maintain ROM and help off-set some of its age-related loss. In general, however, the longer someone waits to start some type of flexibility program after adolescence, the less likelihood of absolute improvement."