When I got 18, I was already a trainee, this excludes you from mandatory military service in germany until you're done. When I got my job-degree with 21, mandatory military service was removed in germany.
But I would not want to join the military anyway. The only time I would pick up a gun for the bundeswehr is when our country is really threatened (inside the country, border - which will only happen if china begins world-conquering). But not to drive around the ocean hunting pirates or getting killed in the middle-eastern deserts.
Also If you kill a man/woman, you take a away someones father/mother, son/daugther, brother/sister, friend or lover. I don't want that to happen to me, so I would never do it to another human.