Mountain House advertises a 30 year shelf life. When pressed, they admit it's 25 years "plus", with at least 5 years past their expected expiration date.
I have a Mountain House "Chicken Teriyaki with Rice" meal in my hand right now. This one was purchased about a year or so ago and has stamped on it "Best used by July 2013". Far from that 25 years. However, I have eaten ten year freeze dried meals several times and they were as good as new from all I could tell.
This package does not have a word about being opened, but I have had other brands that say it's okay to cut a small hole in the pouch when at high elevations as long as the meal is eaten during that trip. The hole is to let the air out, because the pouch seems to get inflated at higher elevations where the outside pressure is lower. They look like they are filled up with air when at 8,000 feet in elevation. The small hole lets the higher air pressure out of the pouch so it takes up a lot less room in a backpack.
-Don- SSF, CA