var age = Math.floor(Math.random()*61)
console.log(age)

if (age >= 10 && age < 20){
  console.log("10代")
}else if(age >=20 && age < 30){
  console.log("20代")
}else if(age >=30 && age < 40){
  console.log("30代")
}else{console.log("それ以外")}