Older people have less elasticity in their skin, so it doesn't absorb the impact of small blows as well as it used to.
The skin is also thinner, which means that bruising shows up more easily.
Bleeding also continues for longer (resulting in larger and more prominent bruising) when people are taking medicines to thin the blood. (It's particularly noticeable when people are taking Warfarin, as my late father used to do, but it can also happen when taking aspirin).
Relevant links:
http://www.medicinenet.com/bruises/article.htm
http://www.nlm.nih.go...cy/article/004014.htm
Chris