As far as I know that information is not visible on your listings although you, as the seller can see it.
The only way for potential buyers to see how long your items have been listed for is to sort them by 'newly listed'. That the shows the date the item was first listed.
You can't remove that information, although most people probably won't sort your items in that way anyway.
You could cancel your listings and relist as 'sell similar' which would start them as new listings but, as a private seller, you do only get 300 listings a month and may soon use that amount.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
