This will be the first post in what I'll call "quick-hitters". Short to the point posts to accomplish a specific task.
So you want to find a quick way to know when the domains in your forest were created.
This is where I really like a tool called ADFIND by Active Directory MVP Joe Richards. Joe is one of my favorite people in the AD world. Really smart but also a cool guy that is always willing to help.
The command is
adfind -gcb -f objectcategory=domain name whencreated -tdcgt
In my example screen shot I only have one domain in that forest in my lab but it works across your forest becasue of the -gcb switch. If you only want to search your domain you can replace -gcb with -default.