Can you provide a definition of old? That way, when I come across another thread, I can know if it is too old for me to post a reply/comment.
Overall, there are two main problems with necroposts: 1) people not realizing that they're replying to people in the past, well after interest has waned in the subject, or 2) their new reply is less about the original subject, and more about their own questions, solutions, accomplishments, etc. In both cases, a new thread (or no thread if the reply really isn't that useful) is a better solution so that things are kept relevant.
When to necro/bump a thread:
For stickies: generally doesn't matter, especially as they're already up top.
For sales/group buy threads: usually these aren't subject that matter too much for bumping when old. Still, if the thread is old, first try PMing the original author to see what is up. If you want to keep the thread in recent memory, bump the thread before the thread is old to keep it near the top. Don't bump sales threads that have completed or those that seem very dead; just PM the original person, or start a feeler thread or something. If the thread was a feeler, or an update has been long waiting, you can bump it, but usually you'd be doing that periodically at shorter intervals anyway. If it's 6-months later, the sale has probably died off.
For others: 6+ months is generally old, but if there is really good information directly related to the post, it could be fine for bumping with a new reply. 1-years old is old. Really, the main idea is whether your reply is still relevant/useful; a thread being old usually means it has lost its interest/relevancy to most people. For example, even for a not-so-old 2-3 months-old thread (measured from last reply), is it really worth a bump just to say "that's cool"? Probably not. I'm sure the kudos would've been relevant when the subject was brand new, but not 6-months after the fact just because you didn't see it at the time it was fresh. You could just rep the person, or PM him/her or something.
If you're answering an old question, just PM the person. After all, they'll be more apt to notice the PM rather than the post that they made long ago and everyone forgot about it (including the OP).
Also, is your reply specifically about the OP's topic, or is it more about what
you have done, or what you are asking? If the latter, a new thread would be better to make a clean break with your info and questions; just refer to the old one if you need to give people background information.
Your post above was more about what
you had figured out and done; it's not that helpful for people who were investigating this subject 4.5 years ago. Even if some people think the subject is interesting, starting a new thread with that information, referring to the old, and maybe posting some other info about your project would be a better idea. You can even just start a new interest thread on the same subject with some points/questions
you're interested in, not just tag it to the end of an old thread.
