Example using a lazy variable as a first class citizen + date formatters!

Person.swift

import UIKit

class Person
{
  let name: String
  let birthday: NSDate
  lazy var age: Double = self.calculateAge()
  
  init(name: String, birthday: NSDate)
  {
    self.name = name
    self.birthday = birthday
  }
  
  init(name: String, birthday: String)
  {
    self.name = name
    let dateFormatter = NSDateFormatter()
    dateFormatter.dateFormat = "dd/MM/yyyy"
    self.birthday = dateFormatter.dateFromString(birthday)!
  }
  
  func calculateAge() -> Double
  {
    let diff = NSDate().timeIntervalSinceDate(birthday)
    
    //estimate of seconds in a year (not including leap years)
    let years = Double(diff) / 31536000
    
    return years
  }
}

let matt = Person(name: "Matthew", birthday: "24/10/1993")

matt.age