A great article on Femur Fracture
Since the femur is such a strong bone with ample protection supplied by large muscle it can only be injured by significant force or by being weakened by age or disease.
Femur fractures are seen most commonly in two age groups. The first age group is individuals that are less then 25 years old. The most common mechanism of injury for this age group is on and off-road vehicular accidents. The second age group is individuals older then 65 years old. While this age group is not the only group to suffer from bone weakening cancer and osteoporosis it is the group that has the greatest occurrence of this problem. Chronic diseases and age weakens the bones and this is reason that this age group is at high risk of femur fracture.
Please see the full article. Femur Fractures 2006.