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Are the pages that are linking to that old domain "indexed". I guess they are, that's how you found them in the 1st place. How much of a compromise is it to use the old-domain ? Is it a compromise ? Personally I have seen varying successes with old and new domains, same topic. There are just too many factors that influence the rankings. I would look at the top 20 websites that are ranking for atleast 5 variations of your primary keyword and check their domain age. That should lay it out for you. Feel free to post the new domain, old domain or PM me the URL in private and I can check it for you.