The igneous rock, if it has pushed its way, as a magma into the surrounding sedimentary rock such as a sandstone or a limestone and cooled off, will have a relative age as younger than the sedimentary rock. Also, if the igneous rock has altered the sedimentary rock at their contact, then the alteration will have a similar age to the igneous rock age.
I would say that the Rocky Mts were the youngest of all these mountain ranges as they were created between 80-55 million years ago during the Laramide Orogeny and also they are generally much taller than the other mountain ranges mentioned here which is a characteristic of young mountain ranges and their features are sharper as they haven't been eroded the way most of these others have so have been subject to erosion for less time than the others.
Because Earth's outer core is liquid and S waves cannot travel trough liquids
basically all because of the coriolis effect Google it
To maintain the same commuting vessal