And in the overall scheme of things, a properly maintained 737-400 isn't that old an aircraft. Plenty of -300s still in service.
In terms of numbers, it is not that old. But when you take into account that these short haul a/cs fly multiple sectors within a day to remote airports with sparsely maintained runways and taxiways, then the airframe has aged considerably from their actual years by their landing cycles and "rough" treatment.