Osteoporsis often called an old disease and rarely occurs in children. The bones of a healthy individual is always undergoing a process of renewal in which the protein and minerals, especially calcium, is stored and re-absorbed. During childhood, when we grow, the deposition of protein and mineral absorption over and around the age of 30 the two processes are balanced and at its peak before the process slows down and starts growing imbalance. If the imbalance is too much for developing osteoporosis.
The risk of osteoporosis can be reduced by changes in lifestyle and food consumption. Diet, regular exercise reduces the risk of osteoporosis. Eat lots of foods that contain calcium, vitamin D, bisphosphonates would be very good for the bones.
Here are some definitions of "what it is osteoporosis" which I quote from some reliable sources:
Osteoporosis is a condition characterized by a decrease in the density of bone, decreasing its strength and resulting in fragile bones. Osteoporosis literally leads to abnormally porous bone that is compressible, like a sponge. This disorder of the skeleton weakens the bone and results in frequent fractures (breaks) in the bones. Osteopenia is a condition of bone that is slightly less dense than normal bone but not to the degree of bone in osteoporosis.
http://www.medicinenet.com/osteoporosis/article.htm#osteoporosis
Osteoporosis means “porous bone.” If you look at healthy bone under a microscope, you will see that parts of it look like a honeycomb. If you have osteoporosis, the holes and spaces in the honeycomb are much bigger than they are in healthy bone. This means your bones have lost density or mass. It also means that the structure of your bone tissues has become abnormal. As your bones become less dense, they become weaker.
http://www.nof.org/aboutosteoporosis/bonebasics/whatisosteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. Osteoporosis is defined by the World Health Organization (WHO) as a bone mineral density that is 2.5 standard deviations or more below the mean peak bone mass (average of young, healthy adults) as measured by DXA; the term "established osteoporosis" includes the presence of a fragility fracture.
http://en.wikipedia.org/wiki/Osteoporosis