Which of the following is considered a "red flag" symptom in patients presenting with headache?

Age over 50 is identified as a "red flag" symptom in patients presenting with headaches because it can indicate the possibility of more serious underlying conditions, such as a malignant process or a new-onset headaches in this demographic, which warrants further evaluation. Headaches in patients over the age of 50 can be associated with conditions like giant cell arteritis or other vascular issues, making it critical for healthcare providers to investigate these symptoms more thoroughly.

In contrast, unilateral headaches, sinus pain, and photophobia can be common symptoms associated with primary headache disorders, such as migraines or tension-type headaches, and are not inherently alarming in the same way as the age over 50. While they can warrant attention depending on the overall clinical context, they do not carry the same level of concern for serious pathology as age does.