The formula, plainly
Healthy weight (kg) = BMI × (height in meters)². Multiply the low end (18.5) and the high end (24.9) by your height squared to get your range.
For a 5'6" adult (1.676 m), the healthy range works out to roughly 115–155 lbs (52–70 kg).
What changes with age
Under 65: use the standard 18.5–24.9 band.
65+: several large studies show a slightly higher BMI (up to ~27) is protective against illness and fall recovery. A 2–3% upward shift in the range is reasonable.
Frame size and muscle mass matter more than age for most adults under 65.
