They can tell it's 15,000 years old by the style of the engravings???
If man is millions of years old as evolutionists claim, there should be a lot more ruins of civilization and signs of agriculture than what we find. It is simply not believable that man existed for millions of years with current cranial capacity and the ability to use tools, but did not discover agriculture or writing prior to the last 15,000 years.
If man is millions of years old as evolutionists claim
Evolutionists claim no such thing. The species homo sapien is generally considered to be about 300,000 years old.
For most of that time, they lived primarily as hunters and gatherers. It is generally believed that they began forming civilizations about 15,000 years ago, though scientists disagree on that number.
Why not? It had to have happened sooner or later; it just happened to have happened later.
You do realize, of course, that there are people living in South America who never discovered agriculture until contacted by explorers?
Yes, with a plus-or-minus 5,000 years error bar.