* Steep sides: Young mountains haven't had much time for erosion to round off their peaks and slopes.

* Sharp, jagged peaks: These are a result of the intense uplift and folding that created the mountains.

* Narrow valleys: Valleys are formed by erosion, but in young mountains, the erosion hasn't had much time to widen them.

Examples of young mountain ranges with these characteristics:

* The Himalayas (Asia)

* The Andes (South America)

* The Alps (Europe)

As mountains age, they become more rounded, with gentler slopes and wider valleys. This is due to the constant erosion from wind, rain, and glaciers.