How to contact a PI when I have been out of the system for a while?
The german system is rather diverse; every PI can do their own thing.

For structured PhD programs (i.e. international Max Planck Research Schools; IMPRS) you will need: one or more publications; and good letters from two or more of your formers PIs. Max Planck is very competitive.

For individual PIs you'll be better off with a personal recommendation by your former supervisor.

Your age will factor in as the time between your last contact with academia and now, not as ``age'' in itself. To be honest, this will make things complicated, especially without strong support from a Prof. on your side.