There are a few factors to consider for old posts:
Has this question been bumped recently anyway (so it's already on the front page)? It sometimes happens to me that Community (or a new answer or edit) bumps something, causing me to see it for the first time (or the first time in a while), and I spot some things that could be improved. It's already active, so go ahead.
Does the post have a serious, accidental flaw, like a link that has gone stale since it was posted, or a MathJax error that produces a misleading result (but you can clearly tell what was meant)? Go ahead and fix those; maybe you're only changing a few characters but they're important characters.
Do the comments contain clarifications from the author (that the author didn't just edit into the post for whatever reason)? Are those comments buried under lots of other comments? Please edit those (and flag obsolete comments); lots of important information gets lost on SE under "show 17 more comments".
When editing, and especially if your edit is going to bump a post, please take a couple extra minutes to see if there's anything else you can fix while you're in there anyway. That typo or ambiguous phrasing in the first paragraph is what caught your attention, but are there other typos you can fix too? Breaking up that wall of text is good; does any punctuation or capitalization need to be corrected too? Is the title clear and informative? Are the tags right? Avoid anything that might be controversial1 and don't impose your own style on someone else2, but do make improvements. Thanks for helping to maintain the site.
1 For example, there is not yet consensus in the English-speaking world about gendered language, singular "they", gender-neutral pronouns like "zie", etc. The author gets to choose; don't override.
2 For example, don't "correct" British spellings to American ones or change consistently-used imperial units to SI.