Not tied to construction year
Smoke & carbon monoxide detectors — required in all residential rentals regardless of age (Health & Safety Code §13113.8, §17926), with owner responsible for installation and testing at each turnover.
Water heater bracing — required on all water heaters statewide, new or existing (Health & Safety Code §19211).
Seismic/soft-story retrofit — driven by local ordinance (e.g., LA's mandatory soft-story program) rather than a single statewide year threshold — check the property's specific city.