OK, now I'm following your syntax....
A hypothesis I have is that if I 301 an existing, old site to it the new site might be viewed as an old, rather than a new one.
It might be more helpful to think of this as re-branding an old site, where you 301 redirect the old domain to a new one. Generally, when you rebrand an old site, you lose traffic for a while as Google sorts out the trust and relevancy issues. Among other things, I assume that Google makes judgments in this interval about whether the inbound links to the old domain are relevant to the new one, and about whether anything sneaky might be going on (as in what you're trying to do).
You don't avoid the issue of establishing trust for the new domain. FYI, this transition period, from old domain to new one, is what was originally called the "sandbox".
The exercise might work if you first simply changed domain names, keeping the same content... then got some backlinks to change to the new domain, and then gradually changed to new content. Depending on how related your inbound links are to the ultimate new content, that might work.
If too many inbound links too fast is going to raise a spam flag at Google, though, why should Google not be careful about a whole collection of inbounds, via a single redirect? And if you pour a whole bunch of inbounds into a domain, whether it's an old one or a new one doesn't make much difference. If you get more backlinks than actual traffic suggests you should be getting, you're most likely going to have problems.
I'm not really bothered about preserving inbound links from the donor site.
The links, IMO, are all you are in fact preserving. It's not the age of the domain.... it's the age of the backlinks, and about a whole set of signals during a transition that indicate to Google that this is one continuous business or entity legitimately evolving into another one, one which deserves the backlink credits.
This recent discussion, btw (whatever the reason for the display of redirected domains in the serps), clearly indicates that Google has the old domains indexed and is following them in some way....
Domain name replaced in SERPS with alias domain name http://www.webmasterworld.com/google/4327200.htm [webmasterworld.com]