How do young earth, Flood geologists explain different age mountain ranges?
Because of the violent nature of the Flood waters, mountains that existed before the Flood or that raised up during the early part of the Flood would have been planed off leaving what appears now to be an ancient mountain range. Those mountains rising during later stages of the Flood would have been greatly eroded, but not totally planed off. They would appear today to be mid-age mountains. The final stage of mountain uplift after the Flood would receive no Flood water erosion resulting in sharper, younger looking mountains.