Age is not a reason
“The site is three years old” describes nothing about whether it works. Sites are replaced on that basis constantly, at real cost and real risk to visibility, without anyone stating what problem is being solved.
Reason 1: the business changed
New services, a different buyer, a repositioning. When the site describes a company you no longer are, that is structural — and it shows up as inconsistent descriptions that models repeat.
Reason 2: it can’t be changed by the people who need to
If a copy update requires a developer, the site will go stale regardless of how it looks. That is a tooling problem worth fixing — who can edit what is a design decision as much as a technical one.
A site you can’t update is already out of date, whatever it looks like.
Reason 3: it doesn’t convert
Measured, not felt: form starts versus completions, which pages precede an enquiry, where people leave. If those numbers are bad, targeted fixes usually beat a rebuild — the numbers worth watching tell you which.
Prefer continuous change
Sites maintained steadily rarely need replacing. The big-bang redesign is largely a consequence of neglect: three years of deferred changes arriving as one expensive project with a launch-day risk.
If you do rebuild, protect what worked
Keep the URLs, keep the specific factual paragraphs that were being cited, and run the pre-launch checklist properly. Most post-redesign losses are self-inflicted and preventable.