I think all of these factors are genetic factor to whether someone develops osteoporosis, but age is the most common factor to cause osteoporosis. Therefore, I think its the most important.
Exercise - can help reduce the risk but its not a guarantee factor.
Gender -can be a major factor due to the higher risk in women for there smaller bones but not as constant as age.
Diet- lack of calcium intake does increase risk but this lack of calcium in the bones can also occur purely through age.
I'm not 100% but with the facts, I'm pretty sure its AGE.
