BMI by Age: How BMI Works Differently for Children and Adults

Last updated: May 2026

BMI by Age: How BMI Works Differently for Children and Adults

For adults 20 and older, BMI categories are the same regardless of age. A BMI of 25 means overweight whether you’re 25 or 65. For children and teenagers aged 2–19, BMI is interpreted differently — age and sex both matter because children’s bodies change rapidly as they grow.

BMI for Adults: Age Is Not a Factor

The standard adult BMI thresholds apply to all adults 20 and older:

A 30-year-old at BMI 23 and a 60-year-old at BMI 23 are both classified as healthy weight. The formula doesn’t change, the thresholds don’t change, and the categories don’t change with adult age.

However, what changes with age is what the same BMI number actually reflects in terms of body composition:

• As people age, muscle mass naturally declines (sarcopenia). An older adult at BMI 22 may have significantly more body fat than a younger adult at the same BMI, because they have less muscle.
• Some evidence suggests a BMI of 22–26 may be acceptable for adults over 74, and that mild overweight may not carry the same mortality penalty in older age as it does in younger adults.
• BMI is specifically not recommended as a sole indicator for adults over 65 — muscle loss means BMI underestimates body fat in this group.

BMI for Children and Teenagers (Ages 2–19)

BMI for children is calculated using the same formula as adults (weight in kg ÷ height in m²), but the result is interpreted completely differently. Instead of fixed cutoff numbers, children’s BMI is plotted against age- and sex-specific growth charts as a percentile.

Why percentiles instead of fixed numbers?

Children’s bodies change dramatically as they grow. A 7-year-old girl and a 16-year-old girl with the same BMI number are at very different stages of development — the same BMI 22 might be perfectly healthy for a 16-year-old and potentially problematic for a 7-year-old. Percentiles account for this by comparing a child’s BMI to other children of the same age and sex.

CDC BMI-for-age percentile categories (ages 2–19)

Example: A 10-year-old boy at the 75th percentile for BMI is within the healthy weight range — 75% of boys his age have a lower BMI, and 25% have a higher one. His actual BMI number might be 17, which would be underweight for an adult but healthy for a 10-year-old boy.

WHO growth reference for 5–19 years

The World Health Organization uses slightly different thresholds for international use:

How BMI Tracks from Childhood to Adulthood

Children’s BMI percentiles do correspond to adult BMI at maturity:

• A BMI at the 95th percentile in late adolescence corresponds to an adult BMI of approximately 30 (obese)
• The 82nd–83rd percentile in adolescence corresponds to an adult BMI of approximately 25 (overweight threshold)

This means high BMI in childhood predicts higher BMI in adulthood — the risk tends to persist. The older a child is and the more severe the BMI elevation, the more likely it is to continue into adulthood.

BMI-for-Age: Practical Summary

Frequently Asked Questions

Does BMI change as you get older?

The BMI categories and thresholds do not change for adults — 18.5–24.9 is healthy at any adult age. What changes is what the same BMI number means physiologically: older adults tend to have more body fat and less muscle at the same BMI compared to younger adults, so the same number can reflect different health risk depending on age and body composition.

Can I use the adult BMI calculator for my child?

No — the adult BMI calculator applies fixed thresholds that are inappropriate for children’s growing bodies. For children aged 2–19, you need a BMI-for-age percentile tool that accounts for the child’s age and sex. The CDC provides a separate calculator for children and teenagers.

At what age does adult BMI apply?

Adult BMI categories (the fixed 18.5–24.9 healthy range) apply starting at age 20. For ages 2–19, BMI-for-age percentile charts are used instead. The transition isn’t always sharp — some clinicians begin applying adult thresholds at 18 or 19, particularly for late adolescents who have completed growth.