How old do you have to be to be legally diagnosed with bipolar disorder?

Any age: If you see a doctor and they diagnose you, you have consented to be evaluated. However, legal or not, that does not mean the diagnosis is correct and you don't have to agree. Get a second opinion and if the second doctor says the same thing, consider trying the recommended treatment.

No age cut-off: Although bipolar disorder tends to appear in the late teenage years, there is no minimum age for the diagnosis of bipolar disorder by a psychiatrist. Here is a link with more information: http://www.nimh.nih.gov/health/publications/bipolar-disorder-in-adults/index.shtml