I'm not a doctor, but -
Inadequate Fat LayerBody fat plays an important role in protecting you from bruises. If you have a thin or inadequate body fat layer, even a slightest bump may cause bruises on legs. Many people lack an adequate layer of fat on their arms as well as legs. Fat cushions cover an impact on the body and prevent bruises. If there is not enough fat layer, one is easily bruised. Thus, you may never realize which causal bump lead to bruises. Thus many thin people, children as well as old people are more prone to bruising for no apparent reason.