ebay/sedo/here at DP you either have to register new and hold for that many years or buy it already aged by someone else
I see many aged domains on GoDaddy's auction site (10+ years) every day. You may want to check there.
expireddomains.net has a list of expired domains and if you're a free member you can search through with the "Birth Date" however there are lots of trashy names, it's quite hard to trawl through that list.
Go to GoDaddy auctions and use the filter they have. Then set the minimum filter (eg. 8 years) to whatever you want to see domains that are at least that old.
Pretty much any auctions or websites like this that sell domains? Do your research though. Don't get sandboxed domains
Go daddy auctions or expireddomains.net, however it still hard but doable to find a gem out of all the trashy domains out there. I'm guessing way back in the day people were quick to snatch up all the good domains for later use. And now... they sell them for gazillion dollars!
once a domain name changes its owner, most indexing scripts reset age parameter to 0. Google worked the same, I don't know if they changed this or not.