Final answer:
To determine the age of something, scientists use methods like carbon-14 dating for organic remains and crater counting for planetary surfaces. These methods allow for consistent age estimations which agree with the age of the universe determined by its expansion rate.
Step-by-step explanation:
We can determine how old something is by using various methods depending on the object's nature and age. For example, the age of the universe is estimated at about 13.8 billion years based on its expansion rate. This estimate is consistent with the observed ages of the oldest astronomical objects.
For dating once-living things up to about 50,000 years old, the method of carbon-14 dating is widely used. In this method, scientists measure the ratio of carbon-14 to carbon-12 and calculate the age based on the known half-life of carbon-14. For inanimate objects, such as planetary surfaces, crater counting may be used, where the number of impact craters is proportional to the length of time the surface has been exposed, given a constant rate of impacts over time.
Both methods, when calibrated and applied accurately, give us a consistent understanding of the timeline of natural history and the development of various objects in our universe.