A website does not have a universal expiration date. Its useful life depends on the business, content, technology, maintainability, and customer experience. A claim that most sites last a fixed number of years needs evidence and is not a reason by itself to commission a rebuild.
Separate maintenance from redesign
Keep factual information, software, backups, and important integrations current. Fix broken forms, inaccessible interactions, and inaccurate pages as they appear. Those tasks may be necessary even when the overall design still suits the business.
Identify what no longer fits
Review changes to services, audience, brand, editing needs, integrations, and customer journeys. Use analytics, support questions, and actual task testing. An unfashionable color palette or an older launch date does not establish that the site is failing.