Assuming that an old domain has no traffic, no backlinks, no dmoz or yahoo directory entries, no pagerank, and no indexed pages, then at the present time there is no real difference. Google does have an algorithm to increase pagerank based on domain age but it is not implemented and might never be.
However, many see old domains as always having some residual properties not available in new domains, which could very well be somewhat true. There are also those that contend that an old domain simply looks better on WHOIS, and that buyers who actually know a little about domains discriminate to a degree against newer names, believing that the property can't be too hot if no one registered it before a certain date which is all relative to the buyer's perspective.
But Rick Schwartz regularly sells handregged domains for big sums of money. So you have a valid arguments on either side of the table. I tend to have old domains but I have definitely sold new ones for just as much.