For OBDII dongle: The latest version of the Mercedes me Adapter app was released on 17 Jan.
For built-in Mercedes me: The latest version of the Mercedes me App was released on 24 Feb, and renamed '2020'.
Technically speaking, you can install the 2020 app even if your car is older and does not have built-in Mercedes me functionality... but it will (obviously) not allow you to do much given that the car doesn't actually support it. And it won't connect to the OBDII dongle either.
The built-in Mercedes me functionality first appeared in late 2013, on cars equipped with COMAND Online. Over the years more and more models got it, including cars equipped with Audio20 etc. It relies on a built-in simcard that allows the car to communicate with the outside world over GSM, initially 3g and later 4g. But cars that don't have the built-in simcard (aka KOM Module) won't be able to connect to the Mercedes me 2020 app.