What age do you have to be to be a passenger in a tractor?

That depends on company policy, what the insurance company will permit, and, in a lot of cases, what your relation is to the driver. If the driver is a parent, then you might be allowed to ride as a passenger under the age of 18, whereas you probably wouldn't be permitted to do so if you weren't a relative. Some companies and insurance companies strictly prohibit passengers - in the case of companies, it's often done as some customers will not allow anyone but the driver of the vehicle into their facility.