how can i find the creation date of one of my apps ?

I don't think you can do this in general on a device. Some apps may include the information on an about page, and if the app is up-to-date you should find the release date in the app store. If you're after the date of first release rather that the current one then I don't think this is available anywhere.

tt2

The closest way that I can find to do this is to check the version history in the App Store on the app description page, but I don't know if those go back to the very first release of the app. I just checked a couple that I have and they show the version history going back to 2010. I just can't remember if that was actually the first release or not.