Why a ship's age usually starts at launch

Why do ship histories usually emphasize the launch date rather than the date the keel was laid down?

Because launch is the moment the hull first enters water and begins its exposed life as a ship, whereas the building-way period is treated as construction before that transition.

The launch-date convention follows the physical change from a protected hull on the building ways to a vessel exposed to water and corrosion. Before launch, the hull remains sheltered within the construction process. Afterward, the vessel has entered the environment that will age it. Launch therefore provides a consistent transition point even when fitting-out and commissioning occur later. The convention also applies to ships launched into fresh water because they have begun their exposed lives as floating vessels.