The Black-capped Chickadees, as evidenced by my banding 12 recently fledged young, have fledged their first broods. Many young birds look very similar to adults, and discerning the ages can be difficult. One method of aging birds is by the ossification of their skulls. Young birds have almost transparent "windows" in their skulls. These windows become ossified and opaque as the bird ages. Observing the relative ossification of bird skulls, however, requires practice and experience.
Tail tips also give a clue to age. Look at the tips of this recently fledged Black-capped Chickadee's tail. The tips of the tail feathers are pointed and unworn. These are relatively new feathers, grown as the bird molted from its nestling feathers (juvenile) to its current basic (winter) plumage.
Compare the shape of those tail tips to those of an adult female Northern Cardinal. The adult bird does not molt all its tail feathers at once--the molt into winter plumage takes weeks and does not occur until after the breeding season is finished. Thus the adult feathers are squarish (not pointed like in the chickadee) and relatively worn.