A simple JavaScript If query. If one "if" is used I don't want the others to be used. If I type 18 the following answers from the other ifs show up

var age = prompt("please enter your age")

var letters = /^[A-Za-z]+$/;

if (age == 18) {
  alert("welcome to our website")
}
if (age > 18) {
  alert("welcome to our website")
}
if (age < 18) {
  alert("you aren't allowed to visit this website")
  window.close()
}
if (age = letters) {
  alert("please enter a valid number")
  prompt("please enter your age")
}