"....while the former group is declining."...

Yes, as we age, we tend to die...

So, how do you eliminate this built-in skewing of the statistics???

How does one define a "Catholic"?  Mass attendance is the norm, but as you've pointed out it doesn't hold up in a younger demographic.  So are the ones no longer attending weekly Mass really Catholic?  According to what definition?  Self-declaration?  Do we limit the definition to outside actions (weekly Mass attendance) or interior spirituality?