How to contact a PI when I have been out of the system for a while? I want to do a biology/biotech related PhD in Germany, I have an MSc in the field and some good ideas about what I'd like to research (biofilms and/or synthetic biology, those really excite me) but in short I'm out of the academic environment from quite some time, I'm in my late 30s as matter of fact. For some reasons I couldn't follow my aspiration back in time but I want to follow it now and at my age you just leave all if you aren't motivated and don't really like what you're going to to, especially with a family.

My question is therefore: how to approach a German group/PI in my situation? Especially considering that I don't really know how the German system works and I couldn't obtain letters of recommendation as often the Graduate School there ask for.

I would like to enroll in a Graduate School(or the equivalent) mainly because I'm coming from outside but what I really need is someone who take care to bootstrap me to the thing and give me the opportunity to pursue some interdisciplinar approaches.
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.