How old does a kid have to be to fly internationally?

Question

from California to Australia?

Answer 1

You mean unaccompanied?  It depends on the airline and the schedule--if there is a connecting flight to another airline unaccompanied minors are not allowed at all.

Usually, 5-11 *must* use the Unaccompanied Minor program (which costs about $100-150 more), 12-17 it is recommended that they use the UA program.  

You could go to an individual airline's website and look for their unaccompanied minors policy.

Answer 2

If you mean how old a child should be before you can take it on a trip without too much trouble, I would say about 5 years old, especially on a long trip like that.

The atmosphere in a plane cabin can be very uncomfortable for a young child, and the long sit makes it even worse.