Here's some things to look for to identify young juncos.
What to look for: Look for adult juncos feeding the young; they may be fed by adults for three weeks or more after leaving the nest. (Source: Stokes Guide to Bird Behavior Vol. III)
Look for these physical characteristics to help identify young juncos:
* streaked neck and breast vs. gray neck and white breast of adults
* can be more brownish overall rather than gray in color
* sparrow-like streaks on head and back
The challenge may not be in differentiating the young junco from the adults, but rather identifying a young junco that is unaccompanied by an adult. Young juncos can be sparrow-like in their look (a little brown job) and in their ground-feeding behavior. Watch for signs of the developing white outer tail feathers, which are excellent field marks of the Dark-eyed Junco. This is shown nicely in the photo above.
To aid in identifying any of the young birds in your own yard, anticipate what you may see based on the adult birds that visit your yard during breeding season. Become comfortable with the adult birds' overall size and shape and feeding behaviors. You'll find that despite plumage differences between young and adult, these characteristics - size, shape, and feeding behavior - will generally be similar and can be of great help in narrowing the possibilities and eventually identifying the young, unaccompanied bird visiting your yard.