For extensions, you can assert that the extension requires a particular minimum SDK (e.g., 21) by specifying a value for the androidMinSdk field. Compiled apps will have a min SDK value that is the maximum of all min SDKs of the components in the app. The BluetoothLE extension does this, for example, to specify a min SDK of 21. As for the core of App Inventor, we have no intention to increase the min SDK above 7 at this time.