I have found information that states with osteoporosis the spine is impacted first (people in their 60’s , the hips become impacted later in life, more in the 80’s. I think it probably has to do with the fact that the mesenchymal area of the vertebrae of the spine is where the differentiation of cells is determined ie…how many and what will become osteoclasts (bone eaters) versus osteoblasts (bone builders). A balance needs to be established for building and keeping bone at good levels. Sometimes it’s also my understanding that within the mesenchyme the cells may become fat cells , not bone related cells at all.

You have noticed a statistical fact. The spine has more trabecular bone and is more vulnerable to early loss. Vertebral segments are shorter subject to more force that is less distributed, and more subject to movement.
Hips are more likely to lose bone over the long period of uneven impact because of slight differences in leg length. In addition, with age comes increased possibility of other affecting diseases, like cancer. In my case, radiation through the hips has altered the bone marrow affecting the bone density. Cancers of the reproductive and digestive systems often cause bone loss through radiation treatment.
Sophie, you are interested in every detail. Your curoisity is wonderful. You even researched this.