Natural latex rubber is derived from the Latex Rubber tree. Being a natural product it has a few enemies that can hasten or even destroy your mask:
Direct Sunlight
Extreme Heat
Excessive Moisture
Oil Based Products
Silicone is not right for this application, but urethane would prove to be the best fit for your mask. A lot of times in the movie industry they use latex do to its inexpensive nature and its ability to cure quickly when baked.
In an environment where you might be in the sun, having it on your face and constantly breathing into it are 4 of enemies that will break down latex. 10 years, yeah maybe if its stored in a dry cool place with no exposure to the sun, but wearing it and sweating into it, it will never make it that long. Just think if you left a rubber band in the sun, it dries up and gets brittle, this is the same thing.
Just making sure your buyers are educated on the materials used in this product. If you dont believe me just google latex, sweat and deterioration. There are plenty of people with costumes falling apart because they are latex.