Domain authority is a tough one, but I will tackle it.
Authority is gained from many sources. The age of a website really starts adding authority after about 5 years. Authority is gained by the sites that link to you and those you link from.
Certain sites are granted authority by Google by virtue of the type of site it is.
For instance:
I have a site for a local politician.
You have a site for a state politician.
Bass Tacker Boats has a site for a member of congress
and Redrubies has a site on White House Jewelry.
Each of us has a page on our site that is PR2.
Even though these pages are all the same page rank, they are going to have a massive difference in the amount of authority they have. This is because in the real world, the White House has more authority than a state politician. They simply do. When someone from the white house speaks, it makes the national news. when Governor blowhard speaks, national media is sorta "Meh"
By the same token, the state politician is going to have more authority than my local councilman. A congressman is going have more authority than a state Governor even.
This works out in any niche, ATT has more authority than a regional carrier. Moz has more authority than my SEO blog.
A big part of it is the authority pouring in as well. Educational institutions have links pouring in from all their alumni, and research partners, making them not only great for PR but also authority.
Authority is a six degrees of separation thing, the closer you are to an authority site, the more of an authority you are. This is the very mechanism that propels link building but people only think of link building as passing page rank. If Moz linked to my site, I will have higher authority, because they are passing me some of theirs. If my site has another site in between me and moz, I will get less benefit. At about 6 sites in between the influence is waning.
It works when you link out too. If you cite authority sites a lot it ads to your credibility within your community. If you link out to crappy sites it does the same thing, except it detracts.
If all that links to me is a bunch of spun filled crappy sites, they are not going to have much authority to pass.
It's like you have to hang out with the better crowd to have a better authority on the web.
None of this is written down in all in the same place anywhere, and some of it is common knowledge (everyone knows domain age contributes), I have just read bits and pieces here and there over the years, and watched my own sites a lot, and this is how I think it works, or at least as best I can articulate it.
Don't consider this Gospel, it's just the ramblings of an old man.
Happy to hear other opinions.
