Yes, adults can develop environmental allergies at any age. Asthma can develop during adulthood as well. A runny nose isn’t always a sign of allergies, though. Older individuals may experience a runny nose due to age-related physical changes -- some people, as they age, develop overactive tear ducts and nasal secretions (it's called cholinergic hyperactivity). Also, some medications taken for other conditions such as high blood pressure, prostate enlargement, or erectile dysfunction can cause a runny nose as a side effect.