Sexing koels is easy - they can be sexed when they fledge (and presumably in the nest). In your birds, the spotty bird in the first picture is a female, the black bird is a male.
To age them, it's useful to know that adults will moult completely at the end of the summer, but birds in their first year usually retain some juvenile flight feathers and wing coverts. Adults also usually have a bright red eye, but in my experience (in Hong Kong), the red eye develops over the first winter and many first year birds have red eyes by the spring.
The age for your male (2nd bird) is fairly straight-forward. An adult would be pure glossy black. The white edges to wing coverts show these to be juvenile feathers. You can also see a contrast between brownish (worn) flight feathers and glossy black lesser coverts. So this bird is a first winter with retained juvenile wing feathers.
Females are trickier to age because the juvenile and adult plumages are similar, although the spots tend to be more buff on juvenile and whiter on adult. On your bird (first picture) the secondaries look worn, but at least one primary looks fresh (black & white not dark brown & buff). There also seems to be similar contrast in the wing coverts between worn juvenile feathers (with buff spots) and fresh adult feathers (with whiter spots). The eye doesn't look particularly bright red, it has a slight grey tone. So I would say that this bird is also a first winter with some retained juvenile feathers.