píolóta
A Phíolóta troda dearg:-
Two elements here:-
1. For the purpose of an FAA PPL, do training hours ever expire? No.
2. For the purpose of an FAA PPL, do training hours obtained in Australia count towards the minimum 40 hours required for the issue of a U.S. licence? Ordinarily, yes.
That's the law. Now let's talk practicalities. While theoretically you need only 13 more hours (of the correct type -- cross-country hours, etc.) to proceed to your FAA flight test (checkride), in reality most of those hours won't help you much. Contrary to what they say about flying aeroplanes and riding bicycles, cockpit skills degrade quite rapidly when not practiced recently. You shouldn't expect to take significantly less time than an ab initio trainee to complete your PPL. You still have to pass that flight test. After a twenty-year hiatus, being ready to do so after another forty hours' training would actually be pretty good going.
One wrinkle to remember: Any endorsements you may have in your log-book from your time in Australia won't be valid for FAA purposes, unless they were made by an FAA-certified flying instructor. So you'll still need to be signed off by an FAA person for solo flight, cross-country flight, and so on.