I think it's not too surprising that the law treats people with diminished capacity differently. It's not a bug, it's a feature, even though it may feel upsetting. There's no winning solution in a case like that.
Well, if the law treats them differently when it comes to punishment, then maybe it should treat them differently when it comes to being able to drive in the first place?
Yup. And we do have some degree of safeguards here-- admittedly, less in California than many other states. They are: physician required reporting of disqualifying conditions, ability for other people to report concerns about capability to drive, and the requirement to show up and undergo vision testing and not flag other concerns in the process.
There's a tradeoff between reducing the very low rate of unsafe driving by the elderly and the burden added to the very old. People over 65+ are still possibly safer, overall, than teenagers.
You'd prefer to throw people with dementia and children in jail? Most of us agree that those outcomes are messed up, and the ability to form intent and/or understand the consequences of one's actions is important information to be considered in formulating whether to punish.
Someone got old and hadn't figured out they were unsafe to drive. It's not the same thing as me choosing to drive 100MPH in a city, operate a vehicle drunk, or keeping going after receiving clear evidence that I'm unsafe behind the wheel.