Average between multiple dates

Hi,

So I have the following table:

And I want to calculate the average date difference, so that the result would look like this:

Identificação         Nascimento 1      Nascimento 2     (....)   Average Date Diference (Months)

PT 04077 1281      5/7/2017             20/6/2016                                12

PT 31760 3837      1/12/2017           null                                           null

I manage to get very close with some attemps, but never quite like this.

I also want that the average only consider the "Nascimento" that have values.

Some "Identificação" have multiple "Nascimento", and thats the hard part, because the number of "Nascimento" varies, with different "Identificação".

Hope that someone can help.