Where the age comes from
Every registry records when a domain was created. OrbitProbe reads that date from RDAP, the structured replacement for WHOIS, or from the registry WHOIS server for extensions that do not offer RDAP, and counts the time since then.
The date describes the current registration. If a domain expired, was deleted and later registered again by someone else, the creation date is that of the new registration, even though the name itself was in use long before.