can't remember a special event or something. we travelled a lot to many different countries when i was young, so i think i already "knew" when i was about 5 or so... think you also realise it pretty well on a ten hours flight with a seat next to the window...

and also it helped that there are 4 different languages spoken in my country. within a 2 hours drive you can reach all 4 parts with the 4 different languages.