Several recent Massachusetts reports of Olive-sided Flycatchers seem to have used the presence of white patches on the sides of the back to age the birds as juveniles. This is incorrect and unfortunately the source of the error seems to be an ambiguous entry in the Sibley Guide – sorry.
Both adults and juveniles can show the white patches, it has nothing to do with age, the patches appear and disappear depending on the bird’s posture. When I painted the images I arbitrarily chose to show the white patches on the juvenile, but then failed to make it clear that adults are equally likely to show them. So please update your guides with the information that the white patches, when visible, are a distinctive feature of both adults and juveniles of the species.
If you want to try to age Olive-sided Flycatchers in the fall it should be pretty straightforward (given a very good view), since they don’t molt until they are on the wintering grounds. This means that adults on fall migration are wearing feathers that were grown almost a year ago, and look worn with indistinct grayish wingbars, while the juveniles are in fresh plumage just a month or two old, and look smoother and cleaner with fresh primaries and more distinct buffy wingbars.