Refers to the date when the site has gone live and seen by the search engines (note that this can change if a domain switches ownership)
The site age is one of the single most important criteria of the Google algorithm,
especially for new sites in competitive niches.
This effect is created by algorithm components rather than an algorithm component itself.
This factor become increasingly more important in the future.
Algorithm based on CTR, bounce rate, time spent on page/site, social bookmarks, etc. that Google collect through use of their sites, toolbar, free wifi, Google analytics, etc. (note that Google has never publicly admitted to use this data)
There's little doubt that the search engines see what users are doing,
especially users that have opted-in to certain search engine products.
I believe Google is still at the learning stage here. Gathering data and try to understand it.
This factor is going to become important in the future.
Link Popularity within the Sites Internal Link Structure
Refers to the importance of internal links pointing to the target page
There are multiple examples of top rankings for competitive keywords with only internal links pointed to it.
Try to use anchor text links with keywords in the site menus and within the content when applicable.
Validation of HTML page code as per the W3C consortium, an authoritative body on the standards of web-compatible code
Good content have noting to do with HTML Validation from pages.
This is and shall probably never be a ranking factor, like it should be.
But the chance is that a part of the text disappears for the spiders when a tag is not closed properly.