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.
Why people check domain age
Age is a quick plausibility check. A shop that claims twenty years of history on a domain registered last month deserves a second look, and a very young domain is a common trait of phishing sites. Buyers of domains look at age as one input to value, and the expiry date shows how long the current owner has paid ahead.
It is only one signal. An old domain can change hands or be compromised, and a new one can be perfectly legitimate.
Domain age and search ranking
Age by itself is not something you can use to rank better. What tends to come with age is what matters: content that has existed for a long time and links earned over the years. A newly registered domain with good content is not held back by its registration date.