I comment on any I find interesting. Some are much older. I think they maybe hard to find because the writer may not have known how to get their article out there better or they were very new at the time with few followers.
Sometimes you find an older hub when you are looking for a very specific topic or keyword. It is worth it to others reading it to still comment; even if the writer is no longer responding.
New hubs are just a fresh way to get immediate comments and responses. If they are in the feed, I am more likely to find them and comment then when I have to search for them.