How can I find the creation date of one of my Apps in the App Store ?

Do you mean an app you have created? Or do you mean an app you downloaded/purchased from the App Store? If you mean the latter, you can go to the app's page in the App Store and tap on "Version History" to see the version history the developer makes available. If the creation date is not there (it may not be for an app that is older and has had lots of updates), contact the developer. Contact information is also on the app's page in the App Store.