years since manufacture (of a car)
Used primarily in the context of used cars to indicate how many years have passed since the vehicle was produced. Often appears in phrases like 年落ちの車 or 年落ちが浅い.
This used car is in good condition because it's only a few years old.
Even a car that's a few years old can be driven for a long time if it's well maintained.
Compound of 年 (year) and 落ち (falling, dropping), literally 'year drop'. The exact origin is uncertain, but it likely arose in the used car industry to describe the age of a vehicle.