I started a bird banding internship this week in northern California. I have been training all week. One of the most important aspects of banding birds is to be able to properly age a bird based upon the plumage that the bird is wearing. In order to do this one must be aware of the different molts that a bird goes through throughout it's life history. We focused on two strategies common amongst passerines; the Complex Basic Strategy (CBS), and the Complex Alternative Strategy (CAS). The vast majority of birds go through a yearly entire replacement of feathers, or a prebasic molt which produces basic plumage. Some birds additionally go through a prealternate molt producing alternate plumage. An example of the latter is the male American goldfinch which goes through a prealternate molt in early spring to produce the vibrant yellow plumage worn during the breeding season.
It is also important to realize that a young bird exhibits slight variations to the molting schedule which can aid in age identification. A bird hatches in natal down which quickly goes through a prejuvenile molt which leads to juvenile plumage. Approximately 3 weeks to a month later, most birds go through the 1st prebasic molt which leads to 1st prebasic plumage. This differs from the adult prebasic plumage in that the flight feathers (primaries and retrices) are retained. So if you have a bird in hand from mist netting and it possesses juvenile plumage in the flight feathers, and 1st prebasic plumage in the rest of the body, then you know that it is a young bird.
This is just a brief intro to some of the techniques available to us to aid us in aging a bird. I plan to describe more once I am more familiar with them. For now, some pictures to enjoy:

White-Crowned Sparrow in Samoa, CA

Black Phoebe in Samoa, CA