If I find an old answer whose code I verified works using, say, Try It Online, can I edit that answer and add the link?
Technically speaking, you can, as one of the official reasons to edit an answer is to
▶ add related resources or links
That being said, here at Code Golf Stack Exchange, we're quite conservative about editing others' posts, and instead prefer leaving comments so the post author can decide what to do about it.
Note that in any case, until you reach 2000 rep, your attempted edits will only be considered edit suggestions, so they need approval by the post author or multiple other users before they become visible to all users.
My suggestion is to check if the post author is still active on the site by clicking their user name and looking for the area on the right saying e.g. "🕔︎ Last seen 1 hour ago". You can also check their recent activity below. If they are generally active as of late, I'd leave a comment saying "Try it online!" so they can copy it. If they have stopped frequenting the site, then go ahead and edit it in.
Edit: Giuseppe makes a good point: don't edit a lot of old posts willy-nilly and bump them to the front page! Feel free to edit an abandoned post here and there as you notice them, but bulk edits are disruptive to the main service of the site.