Can a large corp. disregard state legislation and impose their own guidelines for dependent coverage?

I work for a large corporation with group health. I live in Illinois, where to my understanding, qualifying dependents are eligible for coverage up to age 26. My group insurer says they only insure dependents to age 23. Can large corporations ignore state legislation and impose their own policy?|||No they cannot. However are you SURE that the law REQUIRES that they offer coverage up through age 26? "To my understanding" is not very definitive and it's common for the "common man" to be unaware of the exact wording of the law.