My definition on dead questions are as followed
The user asking haven't been around for long. (Correct answer may or may not exist, but no best answer was chosen by the asker)
The question isn't active for a long long time, no more answers, no more comments.
Is there any clean-up mechanism for these kind of questions? As no best answer is chosen, they are left open even if there is already a good answer to it or answering it doesn't make sense anymore.
For example, this is marked as a dead question to me:
The asker haven't been around for months, and any answers to it will not be marked as best, therefore leaving it open forever.