Normal blood pressure is highly dependent on age and you should take this factor into consideration. For example, a healt adult should have 120/80, children under 12 years - 100/60, juvenile period is characterized by 110/70, people over 50 years have a norm of 130/80 and 140/90 for old people.
Thus, if your blood pressure remains low for years, but a person feels good, you shuldn't worry about anything. In this case, you should only exclude pathologic deviations in your body.