Is there an age limit for a good student discount?

There is no way to say for sure outside of asking your specific insurance company. Most insurance providers that offer good student discounts have stipulations that may make you ineligible due to your age or the fact that you aren't covered on your parent's policy. Your best would be to contact your insurance provider and ask. The worst they can say is "no" and it's even possible that you are eligible and can start saving right away!