The debris ranges in size from dust grains to comets and asteroids tens of kilometers in diameter. When a meteoroid impacts a planetary surface at speeds of tens of kilometers per second it's energy of motion is instantly converted to heat. The resulting explosion excavates a crater. Since the energy of motion of the object is coverted into an explosion the resulting crater tends to be round regardless of the direction of impact. If the impact is energetic enough the planetary surface can rebound , producing a central mountain peak in the crater.
Craters gradually accumlate on an old planetary surface like a sidewalk getting wet in the rain.
All of the planets and satellites are 4.6 billion years old.