There is a fundamental problem with your argument. Creating Adam as an adult might have had practical benefits - but would Adam have been created with evidence of a past above and beyond his existence as an adult ? Is it reasonable to suppose that, say, God would create Adam with a scar from an accident that never happened ?
The Earth and the universe are full of such thinngs - fossils, light from stars that exploded tens of thousands of years ago - and more - ancient meteorite craters, the burned out remains of stars the lava from ancient volcanic eruptions (check out the Siberian and Deccan Traps). The Earth and the Universe are not just "adults" they have the evidence of a long history. Is that evidence a deception ?