BMI is a simple universal formula:
BMI = Weight(kg) / Height(m) ^ 2
For ages 2-20, BMI is compared to the 5th, 85th, and 95th growth percentiles for a given sex, per CDC guidelines.
For adults, healthy range is determined by the World Health Organization (WHO) guidelines. The same ranges are used for males and females, but a slight modification is used for Asian and South Asian populations where obesity starts at a lower BMI.
Yes, BMI is calculated the same for any age, but the definition of a healthy BMI can vary by age.
BMI is 100% accurate because it's just a formula. However, BMI healthy ranges are only a rough estimation and not fully representative of body health. There are normal reasons why a person's weight might be higher or lower than the typical healthy range, such as athleticism or major genetic differences.
To know if your weight is healthy, it can be more useful to look at metrics like body fat % and visceral fat. These can be measured with high precision using a DEXA scan which can cost under $100. Or, you can use a smart scale that estimates body composition using electric impedance, which is less accurate but much cheaper and easier for daily tracking. Guava integrates with scales supporting visceral fat (Withings), and with other smart scales from Garmin, Fitbit, and more.
You can track your BMI, food intake, and many more health metrics over time with Guava.