DS2 just turned 5 and we got the 3 wheeled microscooter for him.
His balance is quite bad, he couldn't get on with one of the cheaper 3 wheel scooters and as for a 2 wheel scooter, no chance. He gets on great with the micro scooter though, he can balance much better and they go quite fast.
DS1 on the other hand got on fine with a cheaper scooter and now races along on 2 wheels, has done since he was 5 (he's 6.9 now).