Source for this? I did some quick digging and this seems to indicate that the Mojave desert has been a desert for several thousand years.

http://pubs.usgs.gov/of/2004/1007/geologic.html

I couldn't find any source for the OP's statement either.

Really wish people would back up their statements with a quick reference when it's proposed as a fact and not an opinion.

The Mojave had extensive low-altitude conifer forests during the Boreal Holocene age, but took on its current characteristics (IIRC) about 8,000 years ago. Poster might be thinking of the cold and wet conditions of the Little Ice Age, which was a periodic climatic anomaly and thus not a useful basis for comparison.