A bruise is an injury, and is basically a little bit of bleeding under the skin. A lot of things can cause it. An injury of course can cause it, but sometimes you get bruising with no apparent injury. This tends to happen as you age for a number of reasons. Your skin is thinner and the blood vessels beneath your skin get thinner, so the shock absorber is less. Also, as you age, you tend to have other medical conditions that require blood thinners: aspirin, coumadin or something for heart attack prevention or stroke prevention. These things thin the blood, which is good for prevention. But also, can make bleeding under the skin happen more easily and makes bruising more common and more severe.