The suggestion is to retrieve the repository creation date in some function from the REST API.
This date can be currently seen in the database, in the table AO_BD73C3_REPOSITORY_AUDIT. The following query returns the creation date for a given repository in a given project:

select ra.DATE from AO_BD73C3_REPOSITORY_AUDIT ra
join repository r on r.id = ra.repository_id
join project p on p.id = r.project_id
where ra.ACTION like 'RepositoryCreatedEvent'
and r.slug like '<repo_slug>'
and p.project_key like '<project_key>'

In the query above, replace <repo_slug> by the corresponding repository slug of the repository for which you want to check the creation date, and <project_key> by the project key under which this repository was created.