You are actually encouraged to comment on older posts!
You wouldn't believe how much this is true. I commented on a post that was 20-30 days old. I figured it'd just fly by and no one except for maybe a couple people would even see it. I check back a bit later and there's people commenting on that 20-30 day old post (with the older posts being 20-30 days old and the newer ones only being a few hours). I honestly think I revived it.
With Reddit, that would've never happened. Most comment would've sat there and been lost forever.