There are of course objective measures of fitness. Most would define fitness as resilience to stress.
This raises the question, "fit for what"? One might expect different levels of fitness of, for example, a Marine on active duty, a policeman on active duty, and an airline pilot on active duty, and so on. In the seventh decade of our lives, we define it as being able to get through our workouts at the gym, do the housework--which has recently involved lifting crates of books and climbing 18 foot ladders-- and pulling the airplane with the towbar. There are surely 20 year olds and 40 years olds more 'fit' than we are. Still, what matters to us is that we can do the things we need to do and want to do.
As for aesthetic surgery, that is a separate matter.