That depends on what country you're trying to ride in.
a 125cc motorbike
Not legally, you have to be 17 to ride a 125cc or less scooter on the road
You can ride a 125cc motorcycle on motorways providing you have passed your test and are riding without "L plates". It isn't the best experience as you are almost constantly red-lining the revs in top gear, but it can definitely be done.
Yes, an A-class.
the lowest age is 16.age:16: 50cc17: 125cc18: any size engine
Well of course not don't be silly...
1300cc isn't a 125cc!! Bell. Id say honda or aprilia rs 125
you take the box were the accelerator is and there is a spring and you move the pin in it
yes --------- Wiki is world wide covering and it's impossible to answer precisely. Where I lives the motorway speedlimit is 130 km/h and with a low-limit of 50'. Any 125cc motorbike can fullfill those demands. But in many countries it's allowed for youngsters to drive 125ccm's, however it's not for sure that those are allowed on the motorway. In Germany it's under demand of 80km/h restrictor that 16year youngsters may drive 125ccm's and i can't imagine such on the autobahn with its unlimited speed.
as longest the i had a drivers course on motor bikes both on road and off and have a motor bike license by both state and (parent optional) yes