When was Hangar 18 Built?

Hangar 18 does not show up in the photos of AREA-51 from 1968. So, the question is when was it built? After the ANP project was cancelled in 1961, the equipment for the nuclear airplane from the Idaho TAN Hangar was shipped to the Nevada Test Site/AREA-51. I believe that AREA-51's Hangar-18 was built in the 1950's and camouflaged using the same techniques that Lockheed used during World War II to hide their factories. If you look at the 1968 photo, it is clear that there is a taxiway going to that part of the base. Why is there a taxiway to nowhere? This looks like evidence of camouflage being utilized.

Edit 21 December 2015
Now that I have done more research. I have realized that there was no need to camouflage Hangar-18. The nuclear engines could simply be put in casks using the Beetle and trucked over the road to the TAN hangar in Idaho or to Area-25's E-MAD building for reprocessing. It is also notable that Hangar-18 appears sometime after 1974. Chronologically this places the construction of the hangar after the "cancellation" of the NERVA program at Area-25. From what we know about Hangar-18 it was used to mount a nuclear propelled version of the Space Shuttle on a nuclear propelled version of the Shuttle Carrier Aircraft. The military's space shuttle was probably a stealth vehicle. 

Lockheed did a very good job of covering up their factories during WWII. 

This picture gives you an idea of how much space there was beneath the camouflage.

In this photo taken by Skylab from 1974 Hangar-18 does not appear. But, the runway has been lengthened to 23,000 feet. So, space operations began sometime before or after this point in time. 

But, then again this photo from 1968 shows that the runway had been lengthened to 23,000 feet. This would be in step with the ASPEN and 5 years after the memo for a secret space lift capacity. So, there was a secret space shuttle in the mid-1960's!  

This photo from 1988 shows Hangar-18. So, Hangar-18 appears sometime before this date and after 1974.

This is the Beetle. It was created to handle nuclear jet engines. It was moved to Area-51 after the ANP was cancelled in 1961.