Germany has a drinking age of 16 (for beer and wine, spirits are 18+), a driving age of 18 (there is assisted driving at 17, where you need to have a parent/relative in the passenger seat until you turn 18), and voting age of 18 for federal elections (state and local elections vary, some states allow 16-yr olds to vote). It was lowered from 21 to 18 in 1970.
Different speed limits for different drivers could result in a "lower-class" driver being an obstruction for others. I don't think that would work out. That said, motorcycle licenses in the EU are separated into several classes with various maximum engine sizes (16+ for 11kW/15hp, 18+ for 35kW/47hp, and 20+ unlimited). They're separate tests.