how to pass this. variable to child function Javascript

Question

The parent.age set to 20 so it will overwrite the "this.age = 5" of the parent function and then I want to pass this.age with the value of 20 to the child function so the console log output should be ( parent Age 20 )

function parent(){
this.age = 5 // this age will overwrite by 20 and then should be passed to the child function

function child(event){
console.log("parent Age", this.age) // <- the output should be "parent Age 20"
}

window.addEventListener('click',child)
} 


parent.age = 20
parent()

console.log(parent.age) // output ->  20