As someone from a “first-world” country, the older I get, the more uncomfortable I am with those phrases. Are they just racist generalisations based on many countries’ post-colonial hardships? Or is there a point to categorising countries? (infrastructural and economic factors etc.)
Technically this refers specifically to cold war nomenclature.... The common misconception is that people use 1st, 2nd abd 3rd world to refer to levels of industrialization. The deeper heart of the matter is that first world referred to countries allied with the US and "the west", 2nd world referred to those allied, or likely to ally with the USSR, while third world was used to refer to neutral or those with no "value" to being allied with.
The TL;FR answer: post ww2/cold war era