Most sources describe plumages as juvenile, 2nd year, 3rd year, 4th year, and adult. However, another source I've read actually describes 6 different plumage variation: juvenal (sic), basic I - IV, and definitive (same as adult). I think we'll stick with the first method here, since that is the most commonly used method. It's probably important to talk a little about molt, as different bird families can very have different molt cycles. Ducks, for example, molt twice per year with each molt taking two to three weeks to complete (and they actually become flightless for a short time as new wing feathers grow). Many others molt twice per year, but don't lose they ability to fly, though they may not fly as well until the molt is complete (classic examples in plumage variation that present challenging ID's are the warblers). Bald Eagles molt once per year, however, the process is spread out up to 6 months. Growing new feathers requires a lot of energy, so spreading the replacement of over 7,000 feathers out over time has its advantages for a large bird - that works out to about 40 new feathers per day. Have you ever seen a Bald Eagle (or other raptors) missing wing feathers and thought to yourself "that poor bird is missing a/some feather(s)"? Though it/they may have been lost in a fight, the bird was probably just molting.
|2nd year plumage|
In the second year the white mottling is quite extensive on the body and wings.This mottling is also highly variable, some birds spotty while others can be almost completely white, especially on the chest and belly. The bill remains dark, as does the eye. Many will begin to show a little more yellow as they get older in the ceres.
|3rd year plumage|
By year 3 they are starting to take on some of the traits that we'll see in adult birds. The eye, beak, and cere are starting to turn yellow, while the head and tail are becoming whiter as the body feathers lose their mottling with white feathers replacing brown.
|4th year plumage|
By year four the eagles are really starting to take on the look of a sexually mature adult. There can still be a little white mottling in the body and wing feathers, the beak, eyes, and cere are nice and yellow. The biggest thing is the "dirty" look to the head and tail feathers as a few brown feathers remain. One more year and they can start looking for a mate!
By adulthood the head and tail are all white, the body feathers nice and brown (notice how most of the feathers on the calling eagle have worn tips that look light brown), while the eyes, bill, and cere are yellow. It can be tricky sometimes, too, when an adult has a dirty head or tail and may look a little like a 4th year eagle - look for body or wing mottling to be sure.
In closing, I also would like to point out that, to be technically correct, in Bald Eagles the term juvenile is used to describe birds that are in their first plumage cycle; "Immature" is the term used for cycles 2 through 4. Have a fun month birding and, hopefully, by this time next month I'll be seeing the first spring migrants here in southern Utah . . .