Nobody can tell you if you are too young to have a child. People start families younger than you. If the rules are such in Germany, than I guess if you want a child of German descent you will have to wait.
If your husband is in the military, will he be around to help you as the your child(ren) are transitioning into your family? If he is not, are you ready to handle it by yourself? If you have to wait until you are 25 in Germany, then you would only have a year left in residing in Germany correct?
Does your child have to be of German descent? There are other countries who might have different age requirements. I would recommend looking at the US State Department's website on international adoption regarding adoptions in Germany to maybe answer some of your questions.
http://travel.state.gov/adopt.html
I wish you luck in your journey.