Because BP varies with age, an easy guideline to remember the lower limits of normal for systolic BP (< 5th percentile) by age is as follows: < 1 month: 60 mm Hg 1 month to 1 year: 70 mm Hg > 1 year: 70 + (2 × age in year) Thus, in a 5-year-old child, hypotension would be defined by a BP of < 80 mm Hg (70 + [2 × 5]).