It's an interesting question. but if you are an old hacker in the US or probably anywhere else in the developed world you qualify for a retirement income. At age 70 it would be something on the order of $2500/month. If you had an unmarried old lady, you would have something on the order of $60K per year plus reasonable health benefits in a few years.
The real problem is shelter. It is probably very difficult to build your shack anywhere near civilization, because of building code restrictions. The minimum, cost is probably some kind of used RV. You could theoretically avoid the code restrictions, because you could drive to a waste pumping station, etc. However if you blend in to a rural community sufficiently to be considered an asset rather than a nuisance, say by continually improving you property, you might get along quite nicely.
Sorry, there is a catch: I'm not actually an old man, i just want to live the life of an old man in a shack. What i really want is to maximize the hours per day dedicated to sleep and whatever else i want to do, which currently is only reading and writing. I had a relatively bad undergrad GPA and basically want to live the life of a professor without having to worry about grad school, then phd, tenure, teaching if i don't want to, meeting with people i don't like, etc.
If I remember correctly, Joseph Campbell waited out the Depression this way. I don't know how he supported himself (probably borrowed the cabin from family), but he basically read books for 5 years.
And what he discovered during those 5 years was really grand. Do you have a reference for his auto/biography ? I know he ran track at Columbia, manage to visit Europe and learned sanskrit.
The real problem is shelter. It is probably very difficult to build your shack anywhere near civilization, because of building code restrictions. The minimum, cost is probably some kind of used RV. You could theoretically avoid the code restrictions, because you could drive to a waste pumping station, etc. However if you blend in to a rural community sufficiently to be considered an asset rather than a nuisance, say by continually improving you property, you might get along quite nicely.