Underweight If you are underweight you’re advised to see your GP. You can also find advice for different age groups from the NHS: underweight children aged 2-5 (NHS Choices) underweight children aged 6-11 (NHS Choices) underweight teenage girls (NHS Choices) underweight teenage boys (NHS Choices) underweight adults (NHS Choices) underweight over 60's (NHS Choices). Next page : Achieve a healthy weight Previous page : Healthy weight