There is evidence from the Moon that is was heavily cratered from about 4.1 to 3.8 billion years ago. What caused this is debated, but one leading theory is the Jupiter and Saturn got into a 2:1 resonance at that time and highly disturbed everything in the Solar System (well, except the Sun)[1]. It is even called the Late Heavy Bombardment, highlighting the unexpected recent age of the event.

[1]https://en.wikipedia.org/wiki/Late_Heavy_Bombardment