Imagine yourself a member of a hunter-gatherer tribe, say, 4000 years ago. You have plenty of time to observe the world around you. In fact, as a hunter or gatherer, closely observing the natural world is essential to your survival.
Every year or two your tribe’s purposeful wanderings return to the same hunting-ground. One of the things you notice is that certain large rocks, which you thought the solidest of things, have come apart, not merely chipped off the edges but sometimes split right down the middle, straight or jagged. You notice this phenomenon in every rocky region you come to; it is very common. Being a wonderer as well as a wanderer, you ponder how rocks come to be split. Surely no merely human agency could do it.
Perhaps you hit upon an answer involving freezing and thawing, or the growth of tree-roots. But then a more complex question occurs to you: you see many rocks broken to pieces large or small, but you never find any rocks put back together. In time, you think, every rock must break up, until the world be made of pebbles. Therefore, the world had a beginning, when all rocks were whole, or perhaps the world was originally just one very large rock.
Given that a few more rocks crack each year, you count them and form, gradually, a rough guess as to the age of the earth. If your tribe has the concept of “billion years”, or “myriads of kalpas”, you think those would be too long a time. But perhaps two thousand years before your birth …
Yes, that sounds about right.
addendum: Most scientists had figured the age of the Earth at billions of years before they could explain why all the world’s rocks have not already become pebbles. The answer has only come in my lifetime, with the discovery of plate tectonics.