Short answer
Look up the domain and read its registration date. The time since then is its age. A whois lookup shows that date and works out the age for you. A domain that expired and was registered again by someone else starts again from the new date, so age is not always the age of the site.
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Where the age comes from
Every domain record has a registration date, set when the domain was first registered with the registry. That date is the start of the domain's age. A lookup shows it and our tool turns it into years and months.
The record also shows when it was last updated and when it expires. Those are not the age, but they show whether the domain is being looked after.
When the age can reset
If a domain is allowed to expire and is deleted, it can be registered again, often by someone else. The registration date then starts over, even if a website lived on it for twenty years before.
Moving a domain to a different registrar does not reset the date. Renewing does not either. Only a fresh registration after deletion does.
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Domain age is not site age
A domain can be old and the website on it new. Buying an aged domain does not give you the history of the site that used to be there. To see the past of a site, look at the public web archive for earlier snapshots.
Look at what the old site was before you buy an old domain. A previous life as spam can follow it.
Does domain age affect SEO?
Google has said it does not use domain age as a ranking signal on its own. Older sites often rank well because they have collected links, content and trust over time, not because of a date on a record.
A new domain can rank. What it lacks is history, so it takes longer to earn the links and visibility an older one already has.
Registering a domain for several years is not a ranking trick either. It signals nothing to Google, though it protects you from losing it.
What the registration and expiry dates tell you about a business
A very new domain behind a shop or an offer deserves more care before you pay. A long registered, regularly renewed domain is a mild sign of a real business, not proof.
Check the expiry date on your own domains. An expired card on the registrar account is the usual reason a domain is lost, and everything on it stops that day.