Most scooters with solid tires are designed for:
- Smooth, man-made surfaces including tile and carpet
- Smooth, unbroken concrete
- Pea-sized or smaller gravel
It doesn't mean that scooters can't be ridden on other surfaces, but the rider can expect a rougher, possibly less pleasant ride.
Scooters can be ridden on grass, but it will always depend on what kind of grass. The better manicured and the smoother the underlying surface, the more comfortable and stable the ride will be. Also, a four wheeled scooter is going to provide a smoother ride over a three-wheeled model because the rider weight is distributed over four wheels.
Should a rider want to ride across more rugged surfaces, they should consider air-filled and larger tires which are used on larger scooters.