关于Swift中的类和结构

Question

struct FixedLengthRange {
    var firstValue = 3
}

class Student {
    var age = 3
}

let john = Student()
john.age = 4

let range = FixedLengthRange()
range.firstValue = 4//this is wrong code

为什么Class常量实例的属性可以改变？结构没有。