class Human(val name: String, age: Int) {
    var age = age
        private set
    
    fun birthday() {
        //party hard
        age++
    }
}

data class Human(
    val name: String,
    val age: Int
)

fun Human.birthday() = copy(age = age + 1)

name

data class Human(
    val name: String,
    private var age: Int
) {
    fun birthday() {
        //party hard
        age++
    }
}

age

class Human(val name: String, age: Int) {
    var age = age
        private set
    
    fun birthday() {
        //party hard
        age++
    }
}