how to tell when ferrets are getting old

hi everyone so i have 11 rescue ferrets and quit a few of them are considered elderly. none of them have any health issues and my oldest who is 7 is honestly one of my most active and definitely the fastest ferret. i luckily have yet to have any ferrets pass away in my care, but i was wondering how to tell when they’re getting close to their time. i’ve been seeing their average life span is about 7 years old but clyde my oldest who is 7 is incredibly active and acts just like my youngest who are less than 2.