The Earth is estimated to be around 4.54 billion years old. The first relatively stable landmasses, known as cratons, are believed to have formed between 3.8 and 3.5 billion years ago.

This period, known as the Hadean eon, was a time of intense volcanic activity and meteorite bombardment. The Earth's surface was likely molten, and the atmosphere was extremely hostile.

The formation of cratons marks a significant turning point in Earth's history. These stable landmasses provided a foundation for the development of life, which is believed to have emerged around 3.5 billion years ago.

Therefore, the Earth was about 3.8 to 3.5 billion years old when the first relatively stable landmasses formed.