I think a great test is grab a 5 year old budget phone with low screen resolution and slow CPU, and hand it to a few people with your app on it for a few minutes. See what they complain about or can't figure out.

If you want to help the grannies of the world, grab an older budget device with a not so big screen resolution and crank the device scale / text size super high.
Man, you would not believe what havoc that causes on most apps.

I know responsive design is hard but damn...