Ava Fertility requires operating systems iOS 10.0+ / Android 5.0+.    

The Ava Fertility app is optimized for iPhone® 6 as well as newer and major Android manufacturers.

If you have an older phone but it is running on an up-to-date operating system mentioned above, then the Ava Fertility app should work on your phone, however the interface may not be optimized for your older phone.