Hi, this is a great question and something I have also been grappling with recently.

My previous research has focussed on adolescent stunting, so thats easy - there is a WHO height-for-age reference up to 19 years. But wasting is more difficult to define. I know some agencies use MUAC<23cm for pregnant and lactating women, so I proposed using this in an upcoming study of adolescent girls, however we later changed to use BMI<18.5 because the ethics committee felt this was more established and used in the (Asian) setting.

Would be great to hear other experience too!

WHO has a BMI-for-age growth chart that goes up to age 19, which may be more appropriate than using the discrete cut-offs of <18.5, >25 and >30 to represent underweight, overweight, and obese for adolescents. As Tash mentioned, WHO also has a growth chart for height-for-age. MUAC isn't a recommended measure to use because there are no reference values.