If you're looking for a serious video editing app for your mobile device, the Vizzywig 4K is the one to get the job done. The app is able to capture and process 4K videos, making the most of the capabilities of the iPhone 5S' camera. The app captures photos at 24 images per second, and then stitches them together to make a video file.
Within the app is a video editor, which lets you modify single frames and add titles, credits and transitions. These videos can easily be uploaded to YouTube, although there likely aren't very many of them yet, since the Vizzywig 4K app by i4software costs a whopping $999.99 for iOS users to download.