What are the Moped laws in Germany?

I was curoius as I am taking german in school, and hopefully when I'm a little older I'll be a foreign exchange student in germany starting at 10th grade and maybe for the rest of high school. Frau S. told us you had to be 15 to get a moped liscense in germany, but I wanted to know things like can you drive on the highways (or auto-bonds) with a moped because cars are expensive. Danke!
Hi! We??re a German biker family, so here the things I know: With 15 you??re allowed to drive mopeds, that??s right. But you have to take lessons (theory and practice) and a written and practical test. It??s pretty expensive, my brother did it a year ago, he spent about 500 Euros and about 4 months from registration to getting his license. The second problem is that mopeds usually arent??t fast enough for our Autobahn and if they are it??s just crazy to go there. With 16 you can get your driver??s license for motorbikes with up to 125 ccm, which allows you, depending on the motorbike, to drive up to 100-120 km/ hour, which is good enough for the Autobahn. But this license is even more expensive and more complicated than the moped. The prices depend on the region where you do it, in General cities are more expensive. With a German driver??s license for cars you are allowed to drive mopeds without further tests or lessons. Maybe you??re allowed with the American one as well. (If they accept it in Germany when you??re under 18)