If implemented properly, the URL should regain its "pagerank" for whats thats worth. I dont believe that the age of the url will make a difference but the backlinks definitely have at least some effect.  You will want to do your best to have inbounds changed instead of redirecting where feasible.

You do lose some juice on 301's so obviously the best course is to contact sites that house the link and ask them to change it to the new URL (once the new URL is live, and you would still 301 redirect).  This isnt always easy to do particularly when there are 1000's of backlinks, so it really depends on how feasible an option that is, and how many backlinks you have (are there only 3? then having an optimized URL will probably be more beneficial than leaving the URL unoptimized, even if you cant have the links changed).

As to 'exactly' how much do you lose? I dont think anyone has a definitive answer. But I have worked with websites that 301 redirect almost every page when they migrate to a new platform and the SEO impact is not severe if done properly.

I still recommend mining your backlinks and having their targets chaged (at least for the more authoratative ones).