Again, remember that operating systems change periodically; if your trip is in 2020, there might be one or two more update that your Phone won't be able to install. The app itself Will update to better work with it, leaving your Phone and many others behind.
It doesn't really matter if others were able to use the app today, yesterday, or if they make it work during their upcoming Christmas trip. For the purpose of your 2020 trip you need to look ahead and update to a newer model, and I agree with whoever mentioned anything from an 8 upwards.