“When should a person with depression seek help?”

If you think she may be depressed, it would be useful to have her see a psychiatrist trained to see adolescent patients for an evaluation. Depression can very harmful to teenagers (and anyone else) as they often have difficulty concentrating, and have difficulty being fully involved in their lives. Depression affects brain function in various detrimental ways. Therefore, it would be important to have her evaluated and, if she is depressed, to have proper treatment.
Thank you for asking.

David H. Lifschutz, MD

An adolescent with depression would best be served seeing their primary care physician initially for overall health screening (including mental health screening). The adolescent may be able to verbalize feelings of sadness, but depression can present in various ways and the person might not feel "sad." S/He may be angry, agitated, have dropping grades in school with poor concentration, have changes in sleep habits and sleep is not refreshing, become sullen and withdrawn, and have a change in appetite with weight gain or loss. The teen often will not inform their parents of suicidal thoughts. They often will find it helpful to discuss this and other feelings with another neutral adult in whom they can confide, and who can offer help (therapy and possibly medicine if needed). 

Susan Sparkman, M.D.  

I would definitely take her to her doctor for an evaluation. Depression is much easier to treat when it is mild before suicidal ideation or a host of other complications arise. While everyone can experience moments of depression, ongoing depression is not normal and has profound negative consequences on self image self esteem and development

Yes, please take your child to be evaluated/assessed by a mental health clinician and/or psychiatrist. Suicidal thoughts are only one of 8 symptoms that defines if a person has depression.