A site's age isn't necessarily tied to the date at which the domain was registered. Instead, it refers to the most recent date at which the site, in its current form, went live.
This means that if a site changed names, niches or formats, the new site age would be based on the day those changes were made.
Changes in monetization method or underlying technology don't necessarily mean that the site has a new Site Established date.
If the site was established at a different URL in the last 12 months, the original Site Established date can only be used when the original URL is included in the sale.
Generally, sites under three months old with no traffic or revenue are called "starter sites" to distinguish from established sites. However, on Flippa, these all fall under the general category of Websites.