I have a few old urls which are more or less dormant. I'm wondering if I add sub-domains with relevant keyword, would the url being old have any effect on the new sub-domain chances of good google ranking.
Yes, subdomains are treated as a separate domain. But Google somehow knows that it is part of main domain... You have to optimize a sub domain in the same way you optimize a domain. However Google representatives say we do not prefer to show results from sub domains.
Yeah! Old domains make good effect on your new sub-domains. Follow and maintain a good SEO strategy will definitely beneficial for site.