[Does really Google sandbox exist? And why some websites get sandboxed?]
First and foremost, there is no sandbox, except for what webmasters visualize in their own imagination.
Second, the use of the term filter is a misnomer. Google applies trust factors that weight the value of some of the signals used for scoring relevancy, however, none of those could be accurately characterized as filters. A filter excludes query results, it is not a weighting factor. Since your web page is never excluded from search results, based on aging, it is a misnomer to call the aging factor a “filter”. It would be more accurate, IMHO, to call the age factor a signal of trust, or a trust factor.
I believe in most cases that when someone perceives the illusion of being filtered or penalized, they have simply enjoyed the temporary benefit of QDF and “feel” like their pages have been singled out for some sort of penalty. This is simply a perception based on a misunderstanding of what is actually occurring.
The suggestion that filters are placed on entire websites is not supported by real evidence unless your website has been de-indexed. Typically, folks who claim this are often ranking well for many keywords with low competition.
I also disagree with your assertion that there is some sort of filtering or penalty based on the number backlinks from article directories, bookmarks or other common sources of backlinks. Having a large number of backlinks may cause your pages to come under scrutiny, but penalties are based on violations of webmaster guidelines. There is no webmaster guideline that suggests that you should limit how much you promote your web pages, only that your methods should be meritorious.