ajohn

It will be difficult to say for sure which is older or younger. Both a juveniles (i.e. birds which are in their first set of true feathers) and both will have hatched and fledged this year.

Probably the best indicator of age in Chinese Pond Herons is the pattern of the wing coverts. In adults the wing is mostly white, in first year birds there are extensive dark areas in the coverts. This is very obvious in Bart's first picture (notice how the wing looks mostly brown), and is also visible in 9Wi's picture. Also, juveniles tend to have finer streaking or spotting on the head than in winter-plumage adults - again, these two birds show this well.

I haven't really looked closely enough at bill colour to know whether that indicates age, but it does vary during the year.

ajohn

Mike's comment is correct in that almost all Chinese Pond Heron adults seem to go into breeding plumage, and most are still in full breeding plumage. This will certainly be the case around breeding colonies.
However, Deep Bay seems to have a very small non-breeding population (first summer birds?) which have not gone into full breeding plumage, and some of the finished breeders seem to have started moulting back into non-breeding. After breeding has finished, separating non-breeding adults from juveniles becomes very difficult, if the pattern of the coverts is not seen.