Your text size should be an effective 16 points when viewed on an iPad - any smaller and the text is too hard to read. Apps that use text smaller than 14-16 points often receive bad feedback through our Support team, or on App Store reviews. Remember that even if you have 20/20 eyesight, chances are not all of your readers do.
Yes, users can employ the zoom function on their iPad however using zoom also adds complexity to the user interface, as the movement of their finger left and right can be interpreted by the iPad as them wanting to navigate to the next or previous section. This can confused users or cause them to get "stuck", giving them a poor experience with your app.
Therefore your InDesign iPad templates should always use 14-16 point text.