https://www.hackerrank.com/challenges/30-class-vs-instance

person.go

package main
import "fmt"

type person struct {
	age int
}

func (p person) NewPerson(initialAge int) person {
	//Add some more code to run some checks on initialAge
	if initialAge <= 0 {
	    fmt.Println("Age is not valid, setting age to 0.")
	    p.age = 0
	}
 
	return p
}

func (p person) amIOld() {
	//Do some computation in here and print out the correct statement to the console
	switch {
        case p.age < 13:
            fmt.Println("You are young.")
            break
        case p.age >= 13 && p.age < 18:
            fmt.Println("You are a teenager.")
            break
        case p.age >= 18:
            fmt.Println("You are old.")
            break
    }
}

func (p person) yearPasses() person {
	//Increment the age of the person in here
	p.age += 1
    
	return p
}

func main() {
    var T,age int

    fmt.Scan(&T)

    for i := 0; i < T; i++ {
        fmt.Scan(&age)
        p := person{age: age}
        p = p.NewPerson(age)
        p.amIOld()
        for j := 0; j < 3; j++ {
            p = p.yearPasses()
        }
        p.amIOld()
        fmt.Println()
    }
}