NEVER.
you see, you ask when you "should" take them. you don't have to, ever. if you can afford chicken breast and other lean protein sources, and had time to cook and eat them, you'd never 'need' a protein powder.
if its more convenient, then sure, take a whey powder with no fancy additives (like NO2, or Creatine, or sugar to make it a weight gainer). age isn't a factor, because its just whey protein...not some magical substance.
protein is just food. protein powder is just convenient protein.
no more than 2 shakes a day though...solid food is always better.