What is the Altitude limit for mosquitoes in Colorado?

Answer:

According to the Colorado State University Extension page on West Nile Virus: http://www.ext.colostate.edu/westnile/faq.html
Mosquitos can live at least to 10,000 feet:
<snip>I like camping and other activities in the mountains. Do the mosquitoes that transmit West Nile virus occur at high altitudes?

Culex tarsalis, one of the main West Nile virus vectors, commonly occurs up to altitudes of 8,500 feet and is found as high as 10,000 feet above sea level. Other potential vectors are more common above 8,500 feet and can be found well above this elevation. On the other hand, the transmission season becomes shorter as elevation increases, which probably reduces risk significantly. Given the lack of knowledge and experience with this disease in Colorado, it is prudent to use an effective repellent when mosquitoes are active, even at these higher elevations.</snip>