When it comes to some animals, it is basically impossible to tell their age. For example, when it comes to birds, they don't really change after a certain age. That's why a method such as bird banding is used. It's basically attaching a band to a birds foot and cataloging it. This is where the fun starts.
Does your family lack an interesting family secret? Don't worry I've got just the idea. Raise a bird that typically already has a long lifespan. When it starts reaching the end of it's lifespan, buy a second bird that looks nearly identical to the first bird. Remember to include the bird in every family picture. Make sure you only ever show one bird in the pictures.
Remember to pass down the bird and the tradition of bird swapping to your children. Eventually your family photo album will be found and cause a commotion. People will be wondering how this random bird seems to have lived for centuries. Just refer to it as your family's guardian deity.
Ps: Most birds require social interaction with fellow birds. Please buy birds responsibly.