Just how dense your bones were at this stage depends on a number of factors: men tend to have denser bones than women and people with larger frames have denser bones than small, slim people. Regular exercise and sufficient calcium in your diet will also have made a difference.

From our mid-20s onwards our bodies constantly repair and renew our bones. Whole parts of our skeletons are in fact reproduced every four to seven years. This process, known as 'bone remodelling', takes place on the bone's surface thanks to two sets of cells - osteoclasts (which destroy bone) and osteoblasts (which form new ones).

From around the age of 40, the osteoclasts become more active and the osteoblasts less active; so more bone is removed and less is formed. This is known as 'age-related bone loss' and it can lead to osteoporosis - particularly for people whose bones were not that dense in the first place.

Healthy bones have a shell of solid bone and an internal honeycombed network of spongy bone. If you have osteoporosis, your bones have lost a certain amount of both structures. This is caused by age-related bone loss but other risk factors such as smoking can play a part.