Hormonal Imbalance: At 70 years old, hormonal changes are common, and an imbalance, particularly an increase in prolactin or a decrease in testosterone, could lead to nipple sensitivity. This is a common issue in older men due to the natural decline in testosterone levels.

Thyroid Disorders: Both hyperthyroidism and hypothyroidism can cause changes in nipple sensitivity. Thyroid function tests would be necessary to diagnose this condition.
Medication Side Effects: Certain medications, such as those used to treat depression, psychosis, and high blood pressure, can cause nipple sensitivity as a side effect.
Dermatological Conditions: Eczema, psoriasis, or other skin conditions affecting the nipple area could cause sensitivity.

Breast Cancer: Although rare in men, breast cancer must be considered, especially if there's a lump, discharge, or other symptoms accompanying nipple sensitivity. A thorough examination and possibly imaging studies are crucial.
Pituitary Tumors: Tumors affecting the pituitary gland can lead to hormonal imbalances, including an increase in prolactin, which can cause nipple sensitivity.
Neurological Conditions: Conditions like neuropathy or nerve damage could potentially cause abnormal sensations, including sensitivity in the nipples.

Adrenal Gland Disorders: Disorders of the adrenal gland, such as Cushing's syndrome, can lead to hormonal imbalances that might result in nipple sensitivity.
Genetic Conditions: Certain genetic conditions can affect hormone production and lead to symptoms like nipple sensitivity, though these would be extremely rare and likely accompanied by other significant symptoms.
Idiopathic Conditions: Sometimes, nipple sensitivity can occur without an identifiable cause, which might be considered idiopathic after thorough investigation.