i need your experience in this... does old aged domain which already has a PR hold its rank after updates??
Nope, if you can't continue to deliver on what it's brought there. I lost a PR2 myself because the domain wasn't available for quite some time because of trouble with a dubious host I used.
If you purchase an expired domain name and put it on another server and change the structure of the website, when Google does the next update on the domain you will lose the page rank, I know this because it happened to one of my PR6 domain names which I purchased with page rank; it is now a PR0 domain.
A domain will not hold its PR if you do not progress the site in some way or another. Say for instance you park the domain names and all of the backlinks become dead then you pr would drop to 0. I highly suggest not buying domains based on PR. PR really means nothing, yes it can say that the site has some sort of authority but they can also be faked.