|Adult Male Budgie|
|Very Young Budgie|
Notice the ring around the adult bird's eye. See the black spots on either side of his chin. In this case, there are two on each side of the bird's head, below his beak.
His nostrils (cere) are dark blue, indicating he's a male. As she matures, a Female's cere will turn brown. The baby Budgie at the right has a light colored cere that will change to either color as it matures. Although dots aren't always present, they are one more sign to check for in identifying a mature bird.
Baby Budgies also usually appear to have very "soft" feathers. It's difficult to explain the difference, but by comparing an adult and a baby together, the differences are noticeable... In the same way that a puppy or a kitten seems to have softer fur than they will when they are grown, there is a subtle difference too in the feathers of young birds. I suppose you could say they are "fresh and new."
Notice, too, that the baby Budgie's eyes are very dark throughout. This will eventually change, but offers a clue — along with the light cere color — that you are observing a very young Budgie.