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I think I've likely got all the value of a brand new $8 domain :), but I'd be happy to be educated otherwise.
Personally, you may want to consider taking both and see which one works better then do a 301....
Well, as I said, it's not really the age, but a clean *history*.
If it's not (clean)/ isn't crawled, has 0 links to it, just forget about it right now.
But if it has been OK all these years, and has at least 1 or two links:
The ups are... crawl-, link- and content ( context- ) history, ie. Google already knows that this domain is about XYZ and won't (re)evaluate its relevance once such stuff appears on its pages. ( People tend to forget that Google is - after all - a search engine trying to find relevant stuff ). You might not rank immediately but will certainly gain ranks much faster for any rewarding activity - including the introducing of related topics. If they match your theme, Google won't throw the domain into the 'waittaminute, lemmecheck' loop of semantic + relevancy / phrase co-occurrence / spam filter examinations every time you add a single word. Or even if it does, the site'll have a pedigree that'll make it pass in an instant. TrustRank might be relatively high ( for a niche site ) with just 10 links pointing to it from day one.
The downside is... if the domain had virtually no activity for years, and all of a sudden it changes its layout, content, pages, URLs and starts acquiring links in a pace that's never been experienced ( if there really was no activity... you know, 0 x 100 = 0 ) that might flag it for evaluation. Perhaps for but a month, perhaps for longer, if the changes are too rapid or too radical, it'll start from scratch as far as relevancy/Trust goes. You'd want to avoid the 'oh,this domain's been sold, huh?' filter. ( OK they don't re-evaluate sites only because of a change in their WHOIS, but that pretty often comes in combination with a change in the theme/targeted phrases... like when a domain expires and the next day a generic MFA site takes its place. ).
On another note, if it has some... just *some* on-topic quality links that match its age... like half a dozen... it'll probably have no problems if you slowly revive it.
I'm speaking from experience btw.
I have in the past 'successfully' flagged age old domains for being changed too drastically, but in most of the cases, the new sites worked like a charm. ( Although even if you mess up the pace, you won't have lasting problems. In that case, you'll need to keep on running forward and ...get more links. )
Just don't alter the original theme too much ( stay close to / start with already introduced phrase sets ), get links in a natural pace and everything's going to be more than fine.
So chosen carefully yes I think aged domains are well worth it.
One thing i would be careful off is changing the domain ownership records if you buy the domain - this could be a problem, but try setting up a contract with the person you are buying the domain from. I have had first hand experience of buying older domain, having the records changed, then in two weeks the site is no where.