When did J replace X?
At what point in time did the "j" replace the "x" in Spanish (I realize we have some carryovers like México and Quixote)?
3 Answers
XVII Century. You can check the article out in Wikipedia.
According to the RAE (Manual de ortografía de la lengua española), evolving pronunciation patterns fomented this change starting from the 16th century onward:
En la Edad Media, la x representaba también el fonema frecativo palatal sordo de dixo, que a partir del siglo XVI evolucionaría hacia el fonema fricativo velar sordo de dijo. Algunos restos de esta grafía se encuentran en topónimos como México, Oaxaca, Texas y sus derivados (mexicano, oaxaqueño, texano...), y en algunos apellidos como Ximénez o Mexia. La pronunciación de esta x, en esas y otras palabras, es fricativa velar sorda, es decir, suena como j; constituye, por tanto, un error ortológico articularla como /ks/.
Manual de ortografía de la lengua española, 1999
Real Academia Española © Todos los derechos reservados
Just to be clear, I think that it might be fairer to ask at what point did j begin to take the place of x in Spanish orthography. That is, it did not occur at a particular point in time but occurred gradually, over time. Such changes, however, were likely cemented (I would imagine) by the formation of the RAE in the 18th century (1713).
Correct me if I'm wrong, but Mexicans stick to the spelling of "Méxcio" whereas folks in Spain prefer or clearly insist on the spelling as "Méjico" - particularly for the reason stated by Gekko and Izano above... very well a cultural thing? 
