svpk's comment aligns with my own understanding. Also, some young-earth creationists may say that God created the earth with age. If you ask them why God did that or where they find such information in the Bible, they'll either say it's part of God's plan or be silent.

Another interesting place where young-earth creationists provide physical theories for biblical events is the Flood. Many ancient cultures have flood myths, some talk of a global deluge in the distant past, and few cultures attempt to derive a physical explanation for the massive flowing of water.

Though less popular today, some young-earth creationists interested in physical explanations (who also almost always have some theories about the Flood) hypothesized the existence of a "vapor canopy", which served as the source of the flood waters. Essentially, the vapor canopy was a water shell which surrounded the earth. The floodwaters came when God caused the vapor canopy to collapse. Now, this hypothesis fails to withstand a small amount of physics inquiry; the earth's gravitational forces would naturally collapse the shell, and the water's shell would make the earth's surface too dark to sustain life. Yet, they're not bad people, just misguided.

Since we're on this topic, there's one physical theory for a global deluge which is my personal favorite from a scientific standpoint. We know the earth has a crust and mantle. The crust is like a hard shell around the mantle, while the mantle acts like a viscous fluid. The mantle is thought to be magnetic [1]. One interesting and obscure field of research is magnetohydrodynamics (MHD), or the study of the dynamics of magnetic fluids. The geodynamo effect [2] proposes that the earth's magnetic field arises from magnetohydrodynamic (MHD) effects in the mantle.

Now, let's say the earth experiences a magnetic pole flip. Presumably, the earth's magnetic field will collapse for some period of time before returning in a reversed configuration. At some point during the flip, the field collapses completely. During that collapse, the MHD forces in the mantle are eliminated. The precise effects are unknown to us, but perhaps the crust then experiences a sudden rotational deceleration. Conservation of momentum says that the earth's oceans will slosh across the surface of the planet. Once the earth's magnetic field returns, the oceans gradually calm again, but much of the earth's surface has now been washed by a tsunami spanning all oceans.

From a purely theoretical standpoint, I like the scientific plausibility of this theory to explain the deluge, particularly because there's no need for the magical invention of a water source. However, there's still issues, like the precise cause of the earth's magnetic pole flip. If the pole flip is caused by, say, the Sun, then we need to know why and if the Sun's own electromagnetic field induces the MHD forces in the mantle. (MHD forces cause magnetic fluids to flow, which, short of additional information, would imply the Sun may cause the earth to rotate via MHD, which, according to a quick web search, is not the scientifically-accepted rationale for the earth's rotation.)

[1] https://www.geoengineer.org/news/magnetic-field-found-in-ear...

[2] https://ui.adsabs.harvard.edu/abs/1978AnRFM..10..435B/abstra...