Content of the rock can be radioactive decay dated. This is used on meteorites. It can show when it was formed, how long ago it broke off a parent body, and how long ago it fell to earth.
Most asteroids date from the formation of the solar system from the dust and gas cloud around the new sun. That was over 4 billion years ago.
Most asteroids date from the formation of the solar system from the dust and gas cloud around the new sun. That was over 4 billion years ago.