This new policy might be a good start, but it doesn't mean anything if Knott's doesn't spend the money to hire more security and retrain all their employees. Even then, I doubt this policy will lead anywhere as it is nearly impossible to determine the age of a minor and aside from getting verbal confirmation from a teen that they are "over 17", not much else they can do to enforce it.
From a PR standpoint though, Knott's had to do something, and they know that there is another take-over event being planned already. They're probably hoping this policy changes on its own, will discourage a lot of kids from coming down there. We will have to see if that turns out to be true.
Either way this will end up costing them a lot of money. Even if they decide to go the cheap route and not hire more security employees, and but the entire burden of enforcement on the poor teenager standing at Fiesta PC, if the policy change on its own, results in fewer pass sales, that could wreak havoc on their budget planning and potentially their long term future.
Its sort of a weird theme park conundrum. If the image of the park is hurt and fewer people want to go, lowering the prices to entice more visitors shouldn't always be the answer.