Cut it's head off and count the rings.
Yes, it is recommended to discard mallard duck eggs that are over 28 days old in the incubator as they are unlikely to hatch at this point. It is best to remove them to prevent any risk of contamination to the other eggs in the incubator.
6-9 months old
according to my study of mallard ducks this is impossible because mallard ducks are all BOYS! By ID1281088566 Ignore ID1281088566, he doesn't know what he's talking about. One of my mallard hens started laying at about 6 months old, in October. They usually don't lay until spring, and I'm sure this hen won't actually hatch a brood until then.
I'm not a doctor but if i were you I'd take him to one.
Mallard ducks are available from most hatcheries. They are inexpensive, easy to breed and quite popular pets. You will need a male an female duck to breed, ad each duck needs to be at least two years old.
It depends on the breed of duck but, I think it's around 15-25 years. The oldest duck is around 27 years I think.
Depending on the breed, they start laying around when they turn a year old.
Thousand-Year-Old Eggs
If a female mallard duck is killed while her ducklings are still young, the ducklings will likely struggle to survive on their own. Mallard ducklings rely heavily on their mother for warmth, protection, and feeding until they are old enough to fend for themselves. Without their mother, their chances of survival may be significantly reduced.
Duck eggs, which are then named "Thousand-Year-Old Eggs".
If you are talking about a Mallard, you really can't until after about 9 months or so. We had two baby Mallard ducks hatch and grow up outside our kitchen window. We just moved them to the park near by after mom stopped showing up and they both look like females. As we have read, this is the case until they are 9 to 12 months old. We continue to visit them and they have about 200 new Mallard Duck friends now. They see us walking down to the bank and swim over, there is no mistake about which ducks they are.