According to the post on Stack Moderators about celebrating the 10th anniversary of the community, the way to determine the age of the site is:
How to determine the age of a site? We agreed to calculate the age by the date of the first question on the site. This is a one-size-fits-all approach that works for all communities.
In order to determine the age of your community, please go to the first question on your site (yoursitename.com/questions/1). For example, https://rus.stackexchange.com/questions/1/
So if you follow the instructions, this is the question that you will end up with:
Which as you can see if you look at the question that it was asked 10 years, 6 months ago.
So we may have missed our 10th anniversary because we were still trying to deal with COVID and all the disruptions caused by the pandemic last year, but hopefully there's still something we can do to celebrate the occasion.
Updates to follow...