The main cause for grey tongue discoloration is leukoplakia. According to WebMD, leukoplakia patches may occur at all ages, but are more common in senior adults.
Leukoplakia is a reaction of the mouth to chronic irritation of the mucous membranes. Other causes include irritation from ill-fitting dentures or rough teeth, sun exposure to the lips, chronic smoking, oral cancer and AIDS. If a doctor considers the grey patches suspect, he or she may request a biopsy. Leukoplakia is usually painless, but may be sensitive to spicy foods, heat, touch or other types of irritation. The condition is often harmless and the patches disappear in a couple of weeks.