You got me wondering so I started to dig around...

It appears that it was enacted in 2009 or earlier. Still digging...

So far..
The most recent Las Vegas zoning code published is from 2009.

19.09.020

The intersection of Main and Fremont Streets shall be the initial point of assigning the address numbers to
all blocks, lots and building in the City. {Ord. 6052 – 08-05-09}

19.09.030

A. All buildings or lots that front on east-west streets which intersect or start from Main Street shall
be assigned address numbers beginning at that point with the numbers one and zero, and
thereafter be numbered progressively through each block. Following each intersection with a
numbered street, the numbering shall begin again with the numbers one and zero, with the
number of hundreds indicated by the number of the intersecting street.

B. All numbers on the east-west streets shall be even on the north side and odd on the south
side.

C. Where Main Street terminates at Las Vegas Boulevard South, Las Vegas Boulevard
South shall be the base line for numbering along east-west streets to the southerly City limits.

source: http://www.lasvegasnevada.gov/files/Chapter_19.09.pdf