Guess technically, 'stone age' is hyperbole. They didn't literally have to climb back up through a bronze age and steal age again.
But point stands, they were severely crippled.
Well, yeah. And it's not exactly a technical term. Don't get me wrong, the strategic bombing was very destructive and killed a lot of people. The way I interpret the phrase is that bombing a country "back to the stone age" means destroying their industrial production. And strategic bombing didn't do that. Germany and Japan's GDPs did not crater; their production of military hardware increased through 1944. I'm sure the bombing kept it from increasing more, but that's not what the phrase implies. When the Allied armies entered Germany in 1944 their opponents were armed with guns and tanks, not bows and spears.
Japan was firebombed pretty ruthlessly. McNamara openly stated that he would have been tried as a war criminal had the US lost the war. Here is a list of 67 Japanese cities that were firebombed with their destruction % and their US size equivalent:
http://www.ditext.com/japan/napalm.html#google_vignette
Source: The Fog of War (Documentary)
I guess this all depends on one’s definition “bombed back into the Stone Age”. I’m sure descendants of those that lived in Dresden might argue that they, in fact, were indiscriminately bombed to the point of ruin.