In the U.S., what's the legal age you can buy anti-depressants?

You can buy anti-depressants at any age as long as you have a valid prescription from the doctor's office. If your 18 you should have no problem going to the doctor's office on your own. Explain your problem and they should give you an antidepressant probably an ssri like lexapro, prozac, zoloft, etc.