What is the youngest age that one could use the PHQ-9 and have it still be considered to be valid in evaluating or monitering depression in children?
The PHQ-9 is a good screening tool validated for depression in adults (it basically utilizes the DSM IV-TR criteria for depression). There is a PHQ-A (adolescents) which screens for moods/anxiety/eating/substance use, validated for 13 – 18 year olds. There are other screening tools validated for children (such as the Reynolds Child Depression Scale) for 8-12 year olds.