A 6-year-old guinea pig would be considered elderly, as their life expectancy is typically around 5 to 7 years, although some may live up to 8 years with proper care. Comparing their age to a human's age can be challenging, as the aging process differs between species. However, it could be estimated that a 6-year-old guinea pig might be similar to a human in their late 60s or early 70s.