Astrophysics first had something to add to the question of ages with the discovery of thermodynamics in the late 1800s.
At some point in the early history of Earth, a planetoid the size of Mars crashed into our planet.
The resulting collision sent debris into orbit that eventually became the Moon.
And it does not work on rocks or thoroughly mineralized fossils; it is only useful for relatively well-preserved organic materials such as cloth, wood, and other non-fossilized materials.
Other methods must be used to estimate the age of rocks and minerals.
“How big” is almost always an easier question to answer than “how old.” Though we can measure the sizes of animals and plants easily enough, we can often only guess at their ages. The ancient Greeks Eratosthenes and Aristarchus measured the size of the Earth and Moon, but could not begin to understand how old they were.
With space telescopes, we can now even measure the distances to stars thousands of light-years away using parallax, the same geometric technique proposed by Aristarchus, but no new technology can overcome the fundamental mismatch between the human lifespan and the timescales of the Earth, stars, and universe itself.
And OE Christians (theistic evolutionists) see no problem with this dating whilst still accepting biblical creation, see Radiometric Dating - A Christian Perspective.
This is the crucial point: it is claimed by some that an old earth supports evolutionary theory and by implication removes the need for biblical creation.
The lowest age defended on a scientific basis is in the 6 to 10 thousand year range.
Evolutionism, of course, requires billions of years to support the plausibility of life's emergence and of subsequent Evolution from “amoeba” to man.
Scientists think that the Earth is 4.54 billion years old.