Almost certainly, there are 4 year olds who ride small motorbikes etc. I think the main problem is that they can't really be held accountable if they have an accident.

16 year olds are bad enough on the road: https://www.hallandalelaw.com/wp-content/uploads/2013/07/IIH...

They also have really poor judgement. A 4yo will find out how high you can get the car airborne.

Animals are born walking. Humans are somewhat deficient and need more training postpartum.