Congratulations!! Washington, DC, to Perth, Australia, with a 13 month old.

IMO, 4-6 months old is a really easy time to fly with a kid. You have gotten over the initial hump, figured out some of the basics (how to feed them, when they are likely to sleep), but you don't have to worry too much about walking or eating solid foods or needing a bunch of toys. They are still light enough to carry everywhere.

The biggest challenge for me was figuring out what gear to bring and what to leave behind, and a lot of that depends on the personality of the particular baby that you get. Mine was a wailing banshee who screamed as though saber toothed tigers were going to eat him if he was not nestled in the arms of another human at all times. On the upside, we never had to schlepp a stroller around.