Those numbers seem extremely unlikely. I would think that almost no 17-year old owns a car and also has two children. That would mean that people with one or no child, or not owning another car, account for only 10% of Ami drivers?
If this is accurate, those 40% under 18 are driving, but not buying. It's probable that parents are buying those cars for their teenage children.
1. half of all Ami _buyers_ have one car and will continue to drive their existing car
2. half of all Ami _buyers_ have at least two children, and those children will the ones driving the Ami
3. 40% of those _driving_ Amis are under 18
4. 60% of those _driving_ Amis are over 18