Can 14 year old children go to the doctor's office without an adult?

Most offices would not see them without an adult. They cannot consent to treatment or procedures like lab draws or vaccinations. Some will see them with a note from the parent but they still cannot consent to vaccines and such.

Exceptions are if it is an issue regarding abuse, drug use, mental health or sexuality (disease, pregnancy, contraception). The exact rules vary by state but in most states the office should see the child in those circumstances without parental consent and often without parental notification.