People with fair skin, older folks, and thin people without much subcutaneous fat will bruise more easily. Like so many variations we see in humans, it is all in your genes. Sudden onset of bruising can be the sign of a blood disorder like leukemia, thrombocytopenia (low platelets) or alcoholism, but other symptoms would also be present.
Talk with your doctor about your bruising and see what explanation fits for you. Since it has been going on since childhood, it is likely to be a manifestation of your individuality.