This is a good question and it is quite confusing. There is a gap for this age group that the WHO needs to address. A similar question was asked here (https://www.en-net.org/question/4099.aspx) and there are some infomative replies.

Since the WHO 2007 BMI-for-age references only goes up to 19.0 years, after 19.0 you can either continue to use that reference as if the person is 19.0 years. or you can use the adult BMI cut-offs (i.e BMI <18.5 for thinness and BMI>25 for overweight)