Here's why:

* Mountain building: Mountains are formed by tectonic plate collisions. The collision pushes up the Earth's crust, creating mountains.

* Erosion: Once formed, mountains are constantly exposed to weathering and erosion by wind, rain, ice, and other natural forces. These forces break down the rock and carry it away.

* Time: The longer a mountain exists, the more time there is for erosion to act on it. This leads to smaller, more rounded peaks and gentler slopes over time.

Exceptions:

* Different rock types: Some rocks are more resistant to erosion than others. Mountains made of harder rocks might be more resistant to erosion and therefore retain their height longer.

* Climate: Climate plays a role in erosion. Mountains in wet, rainy areas tend to erode faster than those in dry, desert areas.

* Uplift: Occasionally, old mountain ranges can experience renewed uplift, raising them up again and giving them a fresh start. This can result in older mountains appearing taller than younger ones.

In summary: While older mountains generally are smaller, there are many factors that influence the height and shape of mountains, including their age, the type of rock they are made of, and the climate they are in.