Thank you for helping to improve site content!
The help center page on editing gives these guidelines:
When should I edit posts?
Any time you feel you can make the post better, and are inclined to do so. Editing is encouraged!
Some common reasons to edit are:
- to fix grammatical or spelling mistakes
- to clarify the meaning of a post without changing it
- to correct minor mistakes or add addendums / updates as the post ages
- to add related resources or hyperlinks
Tiny, trivial edits are discouraged - try to make the post significantly better when you edit, correcting all problems that you observe.
That last paragraph is key especially when you're concerned about burdening reviewers. Don't fix one typo and leave three others; read over the whole post and try to fix anything else that needs attention.
There's nothing about age there. Editing does bump older posts to the front page, so when I'm editing an older post I try to increase the benefit by doing the following:
check the question title and tags while I'm editing the question -- can I improve those too or are they good?
look over the answers while I'm on the page -- is there anything I can do to improve them too?
skim through the comments and flag anything that's obsolete