what speed should they do restricted ??
It fully depends on where in the world you live. Example:
In Denmark there are 3 kinds of scooters:
One is restricted to 30 km/h, and can be driven as 16 years old with a special license.
One is restricted to 45 km/h, and can be driven as 18 years old with a car or MC license.
The last category is Motor Cycle, and these are unrestricted, but usually people don't register scooters as mc's, but instead buy a 250cc or bigger.