The point is about economic mobility - of course, people get tied down as they get older.
Middle class people from wealthy nations who move to poor countries are relatively rich by the standards of their new country. Middle class people from poor countries who move to rich countries are relatively poor by the standards of their new country.
Middle class people from wealthy nations who move to poor countries are relatively rich by the standards of their new country. Middle class people from poor countries who move to rich countries are relatively poor by the standards of their new country.