My sendero is about 4 years old, so they are at least that old. I would venture to guess that they are from the late 90's or so.
Great guns, mine is a 25-06 fluted. Very accurate and easy to shoot.
By the way, are you curious of the name?
Here is Texas, particularly in south texas, the deer hunting is for massive antlers. The land is mesquite brush and cactus. When seismic crews worked the land, we needed to bulldoze "sendero's" for our crews to work on. Usually these are 1 to 3 blades (8 ft) wide. The ranch owners and hunters then set up blinds on the sendero's which are usually quite straight and long. Thus you have set up this nice straight shooting fairway 8 to 24 feet wide by as much as a few miles long. To shoot the deer, it is common to have some very high powered guns to get the reach to pull down a buck at say 300 to 500 yrds from the blind. Hence the gun was named after the common use in shooting.
My source for this was several ranchers in South Texas when I was down running seismic crews and was bulldozing sendero's out for our crews. They were all talking about how our standard design made great shooting fairways. Prior to me coming in and shooting quasi-random slash surveys, the common survey was a mirrored zig-zag so at the right spot you had 6 sendero's crossing at one spot, thus offering the hunter a lot of "lanes" to shoot down.
And then feeders came along and the whole world and rules changed....