A lot of the people who post stuff about looking for players do not end up filling spots quickly due to not running their game here at the store. For this reason I like to leave the threads untouched so that when someone does stumble upon the topic they can still shoot that person an email asking about the game if they chose.
Marking threads as "cold" or "full" is likely to hurt the chances of anyone looking at them or sending an email out to the person running the game. This hurts the game that is looking for players AND it hurts the players looking for games.
If people are concerned with the age of a post they can just take a look, like you said. I believe the date should not discourage a potential player from inquiring about a game he or she is interested in. Maybe I'm alone in my opinion?
It certainly can't hurt to send out a quick email or post to someone explaining who you are, what you're interested in, and when you're available. People have had quite a bit of success with that over time.
Best of luck in your search, man!