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

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. We can say with a high probability that the universe is younger as we look further away. But stars still have time to go through their normal life cycle.