Of course, if older questions have upvoted answers, voting to close them as dupes of your own new question would most likely be unjustified.

But, I say you should feel free to vote those old posts as duplicates of yours, if you have an answer and those posts do not. I would feel no qualms about doing that myself.

Well done for solving an old problem!

Of course, if older questions have upvoted answers, voting to close them as dupes of your own new question would most likely be unjustified. In the case that your post really is better, clearer, more upvoted, it might be OK, but I would definitely ask others for opinions on the case before voting.

However, I say you should feel free to vote those old posts as duplicates of yours, if you have an answer and those posts do not. I would feel no qualms about doing that myself.

Well done for solving an old problem!