Changing post style according to date

I’m sure this should be a relatively simple thing to do, but for some reason today I just can’t think how to do it:

On the front page of site, and category archive pages, I want to style posts that are less than a certain number of days old in a different way to older posts. i.e. display a ‘New’ icon next to them.

Anybody got any suggestions of how to do this?
W
hat do I need to add into the loop to return a difference according to post age?