Unless message author disables replies, anyone can reply to any message even 100 years later.
Those replies are not intended for the original author to read it. Those replies are generally intended for you, who are reading the topic 20 or 100 years later.
Look at these messages, the most popular messages on curezone:
//www.curezone.org/forums/s.asp?f=762&c=0&ob=h
The top message was downloaded more then 1 million times.
Not strange if one out of 1000 people decides to reply.