A 75 year old woman and her 6 year old granddaughter were victims of a train crash. in both cases, t

1)  When humans are in the womb and when they are young bones are actually mostly cartilage.  As we age ossification occurs, primary ossification at the diaphysis and secondary at the epiphysis, Bones also have osteoblasts, which builds bone, and osteoclasts, which "take away bone". As we get older they osteoclasts have a tendency to work more than osteoblasts, due to diet, age, and lifestyle, therefore making the bones more brittle and likely to break.

2) Not exactly sure...  Sorry! 

3)They still exhibit bone weakness due to the lack of gravity.  Part of what helps build strong bones in the use of them and the resistance given when using them on Earth.