A 4-year-old guinea pig can be considered middle-aged, as their average life expectancy ranges from 5 to 7 years. While they may not be considered old yet, it is essential to provide proper care, a healthy diet, and routine veterinary checkups to help them live a long and happy life. Aging in guinea pigs may have different signs, such as changes in energy levels or showing more sensitivity to colder temperatures. Regular monitoring of their health can ensure timely interventions if needed.