Many juvenile birds closely resemble adults in plumage or at least enough so that it is fairly clear what species they are, but not every juvenile bird is quite so obvious ! The below photos are of the same species (identified in the 2nd photo caption)
The above 2 photos are both of Goldfinch ringed on Longis reserve. The first is a recently fledged bird and the second is a male around 1yr old. The immature is also probably male, given it has predominantly black scapular feathers, young females tend to show brown on these feathers.