How old is this baby?

Your guess is as good as mine. Unless you incubate and hatch a chick from the egg, there's really no way to tell. But these hints can get you pretty close to an accurate estimation. 

Nestlings

• If the baby’s eyes are still closed it is probably less than 7 days old. If the bird is weak due to stress or disease, it may also have trouble opening its eyes. It may be featherless or have some down feathers.

• By 12-14 days of age it should have a good amount of feathers coming through (still in pinfeather form).

 Fledgling

• By 16-20 days it should be completely feathered , fluttering, attempting to perch, and interested in leaving the nest.

• By 22-34 days it should be flying fairly well and able to eat on its own although it will still beg its parents for food. 

 Juvenile/Adult

• By 40–50 days its tail feathers should be as long as its parents. But it may still be dependent on its parents for food.

When baby birds are too young to leave the nest, they are referred to as nestlings. When they have enough feathers to fly, the strength to leave the nest, and have begun to perch, they are referred to as fledglings.