I think the combination of two things actually makes this slightly difficult to defeat:

1. The app will take a video and look for movement so a static photo won't cut it.
2. They apparently flash the screen red green and blue which allows them to distinguish reflective and emissive surfaces. So you can't just point it at a video of an old person because no suitable reflective colour displays exist yet.

There are a few ways I can think of to circumvent it:

1. Write an app that displays a video and simultaneously averages the colour of the front facing camera and applies that as a filter to the video, emulating a reflective display. There would be some lag but I bet it works.

2. Use the Android emulator and directly read the screen colour, and use that to filter the camera input (and connect the camera to an AI video). I dunno how detectable the Android emulator is these days though. Probably the age verification apps can detect it fairly easily.

3. Find a homeless person and pay them £2 to look at your phone.