Pawpaw is for dewormer and it's very effective. While, bitter Leaf juice is for treatment of coccidiosis and immune boosting.

At what week should someone administer pawpaw leaves and bitter leaves to the birds? And the durations?

Note that pawpaw leaf is basically for deworming and not encourage at early age. Every medication should show some improvement in 5 days. If not discontinued.

At age 4 above show better results if not organic, but if it's organic at day 1. 

What can be used to prevent or treat coccidiosis?

Many measures. In organic, you can use aloe vera, Bitter Leaf, scent leave plus water leaf etc.

For inorganic - use sulfa drugs or anticoccidial drugs.