True, the earth must be old for Evolution to be true, BUT,
the fact that the earth is old does not mean that Evolution is true. I believe this is called a necessary, but not sufficient condition. Nice try.
The veracity of scientific claims is bolstered by successful predictions. Newton was verified by the appearance of Halley's Comet on schedule. Darwin estimated the amount of time required to evolve from a single celled organism to the observed variety of multicellular life. He noted the current rate of variation and estimated several hundred million years. The correct number appears to be 500 million years.
It's pretty striking that an estimate derived from a biological theory turned out to be closer than the prevailing estimate provided by the physics of Darwin's time.