The term "digger" in Australia is a word for an Australian soldier. It is a respectful term, originating from an awareness of the hardships of the troops in trench warfare etc. Our forces have often had to "dig in".
It is one of Australia's nicer customs that the term is frequently used to address frail and elderly men. If you want or need to say something to a man who looks as if his life is pretty much over, you address him as "Digger" -- as in, "Do you need any help with that, digger?"
Use of the term implies an assumption that although the man may not be good for much now, he served his country honourably in his youth so still deserves respect for that. It is a respectful form of address.
Australia has been involved in lots of wars -- mostly as allies of the Americans or the Brits -- so an assumption that an old man was involved in one of them will often not be astray. Nonetheless many of the men addressed as "digger" will not in fact have served in the armed forces -- but the term is used to convey that the man was once much more than he now is. It is respect for the elderly generally.
I myself normally used the term in addressing elderly men but now I find that I too am on occasions addressed that way if I get into some sort of a pickle. At age 76, I am in fact pretty frail these days so I appreciate that respect and the eagerness to help that goes with it.
And I am even one of those who have some claim on the term. I did reach the rank of sergeant during my time in the Australian army.