In my experience, older listings are more likely to (in order of frequency):
- point to 404s
- already be filled, and nobody deleted the listing
- have received a higher number of applicants over time, decreasing your chances
- indicate an overly-picky employer (why is the position still not filled after 6 months?)
Given that most job sites display listings in chronological order, your company should probably add fresh listings periodically anyways just to rank higher in searches.
The index is updated daily. When it crawls if it notices a job is no longer listed it will pull it from the index. In theory every job on the site should always be available and listing on the company career page. The older ones will still work it's way to the bottom... Search is ordered by date discovered desc.
Some people keep "job postings" up that never close because they are just collecting resumes rather than filling an open position. See justin.tv for instance. I guess that sort of ties into your #3.
- point to 404s
- already be filled, and nobody deleted the listing
- have received a higher number of applicants over time, decreasing your chances
- indicate an overly-picky employer (why is the position still not filled after 6 months?)
Given that most job sites display listings in chronological order, your company should probably add fresh listings periodically anyways just to rank higher in searches.