You’d be surprised!

“The maximum age, as a general rule, is between 3 and 6 years away from the “Compulsory Retiring Age” for a specific role. It is encouraged that applicants conduct research pertaining to their role of interest.”

So its role dependent for Australian Defence Forces, though is worth at least looking into.

Have you considered a lateral transfer?

Some countries, including Canada and Australia, will accept international applicants providing they meet certain criteria. Australia particularly offers pathways to citizenship too.

https://www.defence.gov.au/adf-members-families/military-life-cycle/lateral-recruits