By eight, most riders know which they prefer and are getting properly good at it. The range here reflects that: bigger pro scooters, full-size boards, and commuters for riders travelling further on their own.
The MG3 Zen at $99.99 is the natural next size from the beginner models, with 110mm wheels for more speed and a wider deck. It suits riders roughly 4'4" to 4'10", which covers most eight to eleven-year-olds.
The Kruzer 200 folds down for $69.99 and rolls on big wheels that cope with rough footpaths. The Metro range steps up again with larger wheels for longer distances on tarmac.
The 31-inch drop-through decks are $59.99 and stable at speed thanks to a lowered platform. This is the age most riders can handle a full-size board rather than a mini cruiser.
Prefer actual height over age every time. Children of the same age differ by several inches, and a scooter that fits is the one that gets ridden.