Can an very old thread be re-indexed by Google search engine?

What if someone reply an old thread which was not indexed already.
Can this thread become indexed again?

 

Possibly, yes.

It all depends on if the page is crawled again, the SE bot sees its been updated and add's it to the index.

The affect on the rankings will be very minimal I imagine

 

At here I see almost all new threads can be indexed within couple hours.
Can an old thread has same as the new threads become index again so fast by replying it?

 

I guess it would get reindexed as it would be close to the top of the New Posts page, i.e. it would get a new link.

 

Get a link from another site to that page and it will be indexed for sure.

 

if this page has backlink it will reindexed again

 

Yes.. it will be reindexed.. if you having any confusion.. you can try it yourself by doing that..