Is there any way the fist post can have marking on after a number of days.
Nope.
Threads are generated using original posts made by the author and stored in a database. When you call up a thread, posts are displayed as they are made.
The current system makes a check when you are about to respond by checking the original date against today's date and showing the warning in a new window without making any changes to the original post.
TBH I think it would look horrible if it did something like that. An option I do have is to archive old posts, but tbh I dont think that works well either as all old threads get merged together in one section. I prefer to keep posts and threads in their correct section which also makes it easier if you are trying to look back for a thread that you read a few months ago in say the ISP section and its now merged with say chat posts.
Most people will see the warning and take heed. I dont think its been an issue before and the fact that it has happened a few times - ironically twice in that same thread and the other
time looked like it was made by someone the same day testing out the warning theory.