What causes the blue lines found under the tongue?

The presence of blue lines under the tongue, commonly referred to as "lingual varices," is primarily associated with aging. As individuals age, changes in the vascular structure and a decrease in sublingual connective tissue can lead to the appearance of these prominent veins. Increased prominence and visibility of blood vessels can occur due to factors such as skin thinning and loss of elasticity, common in older adults.

While dental caries, gingival inflammation, and systemic diseases may present with various changes in the oral cavity, they do not specifically lead to the formation of blue lines under the tongue. Instead, these conditions typically manifest through other oral signs and symptoms that are less directly related to the vascular changes observed with aging. Hence, the correct answer revolves around the natural changes associated with the aging process.