The bird in your picture is probably an adult female - the black stripe down the front is very thin and broken, a male would be much thicker especially at the base and unbroken (although best not to say 100% from the photo). It would also explain the duller yellow appearence - I would expect a male to be brighter.
You are correct to say that a juvenile at this time of year would be moving around in a flock with it's parents, siblings and other families. If it was on it's own then maybe not a juv - possibly a dull, worn adult, more likely female? It's the end of their breeding season, so adults may look more washed out at this time of year before post-natal moult is completed.