10 year old tires ok for highway driving?

Tire rubber is an organic compound and it will degrade over time. It doesn't matter if it is stored indoors or outside they will eventually rot. They will last longer indoors but will still eventually go bad. It is called "de-gassing". Go to any motorcycle tire site and you will find that they recommend using no tire past 6 years old.

Sure it may look fine but the chances of a failure go up as the tire gets older.

Chuck