As part of growing up children will get bruises and other injuries, however these are more likely in some areas than others, for example most accidental bruises are seen on bony parts of the body (e.g. shins, knees and elbows). Children can also have fractures of certain bones (e.g. rib) without any bruising.
Bruises cannot be aged accurately, with research suggesting that 50% of people who aged a bruise by its colour were wrong with their estimate.
With any injury, if you are uncertain the recommendation is that medical advice is sought, usually from a doctor or other appropriately trained member of medical staff.
Ideally any injuries should be recorded on a body map marking location, size and appearance.