app.js

var prompt = require('sync-prompt').prompt;

var dob = prompt('DOB (mm/dd/yyyy): ');

dob = new Date(dob);
var today = new Date();

var birthMonth = dob.getMonth() + 1;
var birthDay = dob.getDate();
var birthYear = dob.getFullYear();

var bd = birthMonth + '/' + birthDay + '/' + today.getFullYear();
bd = new Date(bd);

var age = today.getFullYear() - dob.getFullYear();

if(today.getTime() < bd.getTime()){
  age--;
}

var sign;

switch(birthMonth){
  case 1:
    if(birthDay < 20){
      sign = 'Capricorn';
    }else{
      sign = 'Aquarius';
    }
    break;
  case 2:
    if(birthDay < 19){
      sign = 'Aquarius';
    }else{
      sign = 'Pisces';
    }
    break;
  case 3:
    if(birthDay < 21){
      sign = 'Pisces';
    }else{
      sign = 'Aries';
    }
    break;
  case 4:
    if(birthDay < 20){
      sign = 'Aries';
    }else{
      sign = 'Taurus';
    }
    break;
  case 5:
    if(birthDay < 21){
      sign = 'Taurus';
    }else{
      sign = 'Gemini';
    }
    break;
  case 6:
    if(birthDay < 21){
      sign = 'Gemini';
    }else{
      sign = 'Cancer';
    }
    break;
  case 7:
    if(birthDay < 23){
      sign = 'Cancer';
    }else{
      sign = 'Leo';
    }
    break;
  case 8:
    if(birthDay < 23){
      sign = 'Leo';
    }else{
      sign = 'Virgo';
    }
    break;
  case 9:
    if(birthDay < 23){
      sign = 'Virgo';
    }else{
      sign = 'Libra';
    }
    break;
  case 10:
    if(birthDay < 23){
      sign = 'Libra';
    }else{
      sign = 'Scorpio';
    }
    break;
  case 11:
    if(birthDay < 22){
      sign = 'Scorpio';
    }else{
      sign = 'Sagittarius';
    }
    break;
  case 12:
    if(birthDay < 22){
      sign = 'Sagittarius';
    }else{
      sign = 'Capricorn';
    }
}

console.log('You are ' + age + ' years old. Your sign is ' + sign + '.');