I am a 39-year-old french man. I don’t like the Deezer website and app to use “tu” to address me, I would like it to use “vous”, because to me “tu” is used to address to kids and to very relatives, which I’m not with Deezer.
The formality concept, actually exists in most of the languages, and I think many non-kid, non-teenager users would prefer to be addressed in a more formal manner.
The choice of formality could be based on the birthdate of the user, which is set in the user settings, or - better and less discriminating - on a new parameter in these settings.