Some ducks do eat their own poop.
I've seen some ducks eat their own poop at a park here where they have ducks and after the duck pooped it would turn around and eat it.
Then another time I saw another duck come over and eat another ducks poop.
However not all ducks will eat their own poop or others.
Ducklings may also eat their own poop sometimes as well.
The lifespan of domestic ducks is between 5 years to 10 years depending on the type of duck.
Some ducks live longer than others depending on the type of the duck.
Mallard ducks live on average of 5 years to 10 years but they may live a bit longer.
When ducks are in the wild they live at most 5 years but the wild duck may only live as long as 3 years if it becomes unhealthy or gets hunted and killed.
Ducks live much longer as pets or in captivity than they do in the wild as long as the duck is taken care of properly and fed properly.
In the wild the duck may be more prone to disease and may get killed by either hunters or other predators.