It is the level of maturity that is more important than the numerical years
Down here the biggest killer of children under 10 is now Quadbikes.
Your hipicampus ( must be spelt wrong ) is the part of the brain that assesses risk.
On girls it is fully developed by the the time they first menstrate ( same bunch of hormones ).
On boys it usually takes till the mid 20's through to the early 30's
This is why armies were made from young men who will do as ordered without assessing the risk of injury to themselves first.
Also why most idiot stupid accidents happen to boys.
There are also two important mechanical considerations.
1) the child will need to be restrained by some sort of seat belt
2) as you need both hands to steer their feet need to be able to contact the foot board so they can support themselves