MedHelp doesn't close threads merely for being old, so it makes lots of sense to check the date of the original post to make sure you are not pouring your heart into answering someone's question only to then find it is from years ago. But if the question has come up to the top of the feed, as noted, it also indicates someone has recently posted onto the thread, and especially if it is an interesting topic it might mean they are adding something new to the discussion, which might be worth responding to even if the top of the thread is really old. (Of course, it also might mean they simply answered the post not knowing was an old post.) It's always worth checking the date of the original post, but also even if you find the original post is old, it can be worth looking at the latest posts on a thread that has suddenly popped up, to see if there is something new in the discussion that makes it worthwhile to respond.
Also, when people respond to an old post, it brings it up to the top of the list. It's important to always look at the date of when the person posted it originally to make sure it's recent. The date appears next to the person's username on the post.
Hi - questions in each community are listed by those which have the newest activity. So, a thread near the top of the list will be the most recently active. When you open a thread, the dates of the original question will display near the member's name - same with the dates on the replies. Does that help?