Do old fish behave differently then middle aged fish?

Not only can they behave differently, they can look different too. Some fish will move more slowly, some fish may appear to be "resting" more often. Some fish colors become more vivid with age, some...

As old as a human has to be to be a human.

16

Until their 3rd birthday then they pretty much lose all ability to pick-up other fish and find themselves 'fishing' for 'bottom of the barrel' barnacles at skeezy old bars.

if you want to know how fish behave is you can look at YOUR goldfish behave and write it down and that's how you observe fishes.