Aged domains are available in most aftermarkets and in forums like here at namepros. If your considering buying a domain name and want to see how old it is, a comprehensive whois service like - whois.sc - can tell you the original date of registration: just note the "date created" field within the information.
The age of a domain name is only "reset" if its allowed to expire and is subsequently dropped/deleted back to the open registry.
The only case were domain loses its aging its when it drops and its available again to register.
If you buy a domain a Godaddy expiring auctions or Namejet Prerelease auctions then domain age is preserved, same thing happens if you buy a domain from Namepros or another marketplace and its pushed or transfered in your account.