im not a search engine expert but..
it doesn't matter how long a page is on the web.
what does matter is having valid well structured pages.
also having popular sites link to you.
if you are deleting pages the search engine will still link to the deleted page until the robot re-visits your site.also new pages wont be index until its re-visited.
there is a meta tag...
<meta name="revisit-after" content="3 days" />
set the days to whatever you want.im not sure whether this works or how well it works.