Service life
The number of years a home system is expected to last before replacement.
Service life (or useful life) is the typical span a component lasts before it needs replacing — roughly 10–12 years for a water heater, 15–25 for an HVAC system, 20–30 for an asphalt roof. Knowing where each major system sits in its service life is the key to budgeting: a furnace at year 18 isn't a problem yet, but it's a replacement you should be planning for rather than be surprised by.