When did Physics StackExchange launch in history?
One handy place for this information is the user page of the Community ♦ user, which gives the bot's inception date as 1 November 2010.
For more information, you can go to the original Area 51 proposal, which gives a detailed breakdown of the milestones:
- The original proposal was posted on 2 June 2010
- Private beta started on 2 November 2010
- Public beta started on 9 November 2010
- The site graduated from beta status on 24 February 2011 (source)
The list of Stack Exchange site shows some statistics about each site, including its age. Physics launched six years ago, apparently in November of 2010.