After 6 years one could then live fairly well in a relatively poor country on 15k.

Additionally, there are ways to get people to pay for your living costs in very poor countries. If you live in the US and are looking for a fun but not too easy early retirement, spending two years in the Peace Corps is not a bad way to go -- if you do want to spend a few extra thousand on living expenses it will go a lot further than it would in America, and if you just want to let your retirement funds gather a few years of additional interest you can do that. The PC does take married couples and loves people with college degrees and work experience. No kids, though.