What I feel makes it confusing is that it is not clear how you can have 10 years of volunteer firefighting at the age when you are eligible for mandatory military service.

I believe that the other commenters said that you could "commit" to being a volunteer for 10 years and thus could avoid mandatory military service. This is the part that was not clear.

How it worked was: you got mustered for the draft at 17 or 18, and depending on whether you finished school you got your draft notice between 18 and 20. At any point you could declare being a conscientious objector and apply for a spot at THW, firefighters or some care org. Spots at THW and firefighters were rare and sought after, and especially during the cold war you only got them through connections or with special reasons (important job you couldn't interrupt for en-bloc service). You also couldn't just apply to any village fire brigade because there was a mandatory number of exercises and trainings many of them couldn't provide. When you got accepted for a fire brigade or THW, you went to trainings and exercises. Every few months you had to report signed confirmations of those trainings and exercises. If you failed to report in time and sufficient number, because you didn't care, were unable, got thrown out, you could be drafted for military or civil en-bloc service after all.