from my experienced old site + good content gives a big plus on SERP, but i don't know about the old domain. I'm still in the process of experimenting with old domain.
i try to buy as many old domain names as i can with the highest page rank possible. i then make a blog with each domain and monitize it with affiliate ads and google search box.
We can always spend time to improve the contents. But it is not easy to get an old domain with a good name (keywords) related to your field. Giving that I have already had a new domain with my targeted keywords. What I'm trying to understand is what other factors have more weight than "Age". I then work hard on those factors.
Without knowing the search engine algorithms, it's difficult to draw hard-and-fast conclusions. It is generally held that older sites tend to get an advantage. However ... that advantage may be primarily (or totally?) a result of the fact that the older a website is, the longer it has been ... • adding new pages • accumulating new links • and improving the pages' SEO — for example, by tweaking the keywords for greater effectiveness. In sum, there may indeed be an "age-of-website" factor in the algorithm. Or the older site may simply be scoring higher because it has had more time to add new pages and links and to improve the pages' content. These factors should help in the SERPs.