Sounds like they're making a couple of assumptions themselves. 1) The stars that produce a 1a supernova are the same age as the majority of the other stars in its galaxy. 2) Stars that produce 1a supernova are younger as they get further away.<p>We can say with a high probability that the <i>universe</i> is younger as we look further away. But stars still have time to go through their normal life cycle.