The frontal sinuses are the second largest sinuses. They are located in the middle of the forehead. They may not be visible at birth. As they grow, they usually become visible by ages 4 to 6. Frontal sinuses reach full adult size by age 20. About 10% of adults have either no frontal sinuses or ones that are underdeveloped (hypoplastic frontal sinuses). The blood supply from the frontal sinuses drains into veins around the eyes. Infection in the frontal sinuses can spread into or around the eyes. Frontal sinus pain is often felt above the eyebrows.