Hello Anonymous 502
Weight-for-height was never used to assess the anthropometric status of adolescents using the old WHO/NCHS growth references, it was only applied to girls <10y and boys <11.5 y.
Weight-for-age was applied to all children up to the age of 18 y, but in the new WHO growth references it is only applied to children up to the age of 10 y.
The only indices that are now calculated for all children from birth to 19 y (228 months) are height-for-age and BMI-for-age. BMI-for-age is the only index that incorporates body weight, but it is greatly complicated by the fact that BMI is expressed as kg/m2 and the z-score is based on values of BMI for children by sex and month of age. Weight-for-age and BMI-for-age are hard to compare. You can find a comparison of the NCHS and new WHO growth references for older children and adolescents in the American Journal of Clinical Nutrition, 2011, 94:571.
The new WHO growth references for children from 5 - 19 y published in 2007 were based on the old NCHS data treated using new statistical methods, they were not based on new data for this age group. The curves of the index of BMI-for-age were adjusted statistically so that a z-score of BMI-for-age of +1 roughly meets at 19 y with a BMI of 25 for adults, who are classified as overweight, and a z-score of BMI-for-age of +2 roughly meets at 19 y with a BMI of 30 for adults, who are classified as obese. The intention in designing the new reference curves was, I think, that BMI-for-age and BMI converge at 19y and BMI-for-age is the index of overweight for children. BMI alone should only be used to assess overweight in adults. BMI and BMI-for-age both suffer from issues related to body shape and muscularity rather than just adiposity, so they need to be treated with caution as a basis for judging overweight.
This means that different indices of overweight and underweight are used for children of different ages: weight-for-height for children <5 y and BMI-for-age for children from 5-19 y.
No index for age will be useful if age is wrong. We found evidence in Ethiopia that when children were enrolled in primary school their age was set to the official age at entry, 7 y, probably because children were 'grown' enough to be enrolled in school. There is also evidence from surveys that children who are stunted enrol late in school, so have a negative age-for-grade score: a child enrolled in the right grade at the right age has a score of zero; a child who is 1 year late has a score of -1, and so on. There may be a negative correlation between age-for-grade score and z-score of height-for-age (see Social Science and Medicine, 1999, 48: 675).