Visible and buried impact basins, seen as "Quasi-Circular Depressions" (QCDs) in MOLA data, provide important new constraints on the age of the Martian lowlands. The buried lowlands are no younger than Early Noachian, at least as old as the oldest exposed (visible) surface units in the highlands. A model absolute age for these buried lowlands is 4.04--4.11 GY (or earlier) but similar model ages for the largest lowland basins are older yet, 4.08--4.18 GY. The lowland crust both formed and became low no later than 500 million years after Mars formed, and likely even earlier. This constrains models for the origin of the fundamental crustal topographic dichotomy on Mars. Mechanisms which operated both early and quickly during the earliest history of Mars (e.g., large impacts) may be more likely than those requiring extended periods of time (i.e., endogenic models).