One verse most often used to justify a young universe is found in Mark 10:6 : "But from the beginning of the creation God made them male and female." Here "the creation" is understood by many creationists to mean the creation of the universe. Since the creation week, according to the Bible, occurred recently, this would make the universe young. However, the Bible also suggests that there may be multiple creations. Hebrews 9:11 states: "... not made with hands, that is to say, not of this building [creation]." This verse suggests that there may be other creations. Also, Is 45:12 gives further insight:
I have made the earth, and created man upon it:This verse presents the concept that the earth was "made" (formed) but man was "created." Thus the creation can refer to the creation of life on earth. The second part of the verse switches the view from earth to heaven, not necessarily in chronological order, but as a literary device of passing from the lesser (earth) to the greater (heaven and all its host).
I, even my hands, have stretched out the heavens, and all their host have I commanded.
According to this interpretation, the "beginning of the creation" in Mark 10:6 could refer to the creation of life on earth. It might be possible that the universe existed long before. Even the earth might have existed in some primordial form, without life and very much unlike the current earth, before the creation week. During the creation week, the surface of the earth was drastically reshaped, and the earth as we know it came into existence. Thus it would be correct to say that the earth as we know it is young.
This viewpoint has some implications for radiometric dating. If one believes the universe is young, then one has to account for the fact that many dating methods agree on meteorites and give ages of about 4.5 billion years. This requires one to hypothesize that some of the physical constants of the universe have changed. If one believes that the universe is old but the earth is young, then it is not necessary to assume that the physical constants have changed, at least not since the very early days of the creation. One can explain the old ages for rocks on the earth as dates partly inherited from the primordial earth, or as due to decay that was accelerated during the flood. This accelerated decay would not necessarily result from a change in the physical constants, but might result from the earth being bombarded by various kinds of radiation during the flood, possibly gamma rays or neutrinos. If such radiation can accelerate decay processes, it likely would not cause all decay processes to accelerate by the same amount, in keeping with the fact that different dating methods on the earth are typically discordant.
To me it is unsettling to assume that the Lord changed decay constants to cause the flood, as assumed by D. Russell Humphreys in the RATE study, Radioisotopes and the Age of the Earth: A Young-Earth Creationist Research Initiative (ICR and CRS, 2000). As far as we know, the Lord works through natural processes. Did the Lord change physical constants to cause the destruction of Sodom and Gomorrah or the crossing of the Red Sea? It seems doubtful because of the many side-effects that would have been caused. Then why would He use such alterations to cause the Flood? Rather, it seems more reasonable that these interventions were caused by the actions of angels or God Himself exerting forces on the physical world.
One way to accommodate a young earth and an apparently old universe is to assume that somehow time ran faster during the creation week, so that billions of years passed in the rest of the universe during only one week on earth. This assumption gives the earth a special position in the creation of the universe, which does not seem to agree with the observed fact that the earth is just a tiny part of the Milky Way Galaxy and this is just one of billions of galaxies in the universe. The earth does not seem to hold a special position in the universe and it does not seem reasonable to assume that the earth held a special position in the original creation of the universe. However, during the creation week, which could have been much later, the center of action was the earth.
If one accepts Genesis 1:1 as referring to the creation of the universe at some time in the past and Genesis 1:2 to the reforming of the earth during the creation week, then many problems are solved and it is not necessary to assume a change in the speed of light or in the decay constants to explain observed physical phenomena. Thus the statement "In six days the Lord made heaven and earth" (Exodus 20:11) would refer to the reforming, or making, of the earth and not to its original creation from nothingness.