If somebody hasn’t posted in ages, then it’s much less likely that anybody will be looking at their profile in the first place, so I wouldn’t think it would matter much. And for anybody who does look at the profile, it’s probably more useful to see that they made their last comment nine months ago, than just to see that they haven’t commented in the last week.
just make a button saying recent comments, and if it`s ancient history, there should be a line saying no recent posts in the last month, showing the last 10 most recent comments, something like that.