Are there any accepted conventions anyone can think of for showing the age or staleness of a particular row in a set of results on a web page?
I'm looking for some visual way to identify how long something has been in a list.
I doubt there are any accepted conventions, but there are some good examples which may or may not be acceptable for your solution.
Organise the rows so that the most recent is at the top.
Add a column which shows the age.
Represent the age differently based on its age, e.g. the older it is the less granular the age becomes:
Stack Overflow does this for your posts and edits.
A way I would like to suggest, is using a background color for the row that can indicate the staleness. The older an item, the less prominent it should be (I guess from the question).
So I would work with a background color that matches the background of the page when it's the most stale row. And more oustanding/striking when it's fresh.
(Sorry, I can't really find an example immediately)
You've mentioned two different attributes here in age and staleness, which might share the same values or be completely different depending on the type of content you are working with.
As far as age is concerned, there is only the value which continually increases depending on your reference point (e.g. creation date or published date), and this makes sense since you can only be of a certain age. Depending on how specific you want to be and the nature of the website, you can as one of the answers indicated use weighted (more precise for more recent ages) or linear time values (same value regardless of how recent or old it is).
When it comes to staleness (I am more familiar with the term 'freshness' or 'recency') this is a value that can fluctuate so you have to be mindful of how it is used if you also decide to show the age of the web page. My suggestion is that you can apply the same type of styling for both sets of values, but use one of them in a more prominent area (or highlight more) so that it is treated as the primary sort key, and then use the other as the secondary sort key.
Personally I think colours are not as clear in representing the age of something, but if you use a darker or brighter shade of one colour for more recent items and a lighter or duller shade of the same colour for older/staler items then it might be more meaningful. Personally I think showing the actual value is the clearest way to indicate both types of values.