It's just a maxim, it doesn't apply in every case. I think the motivating example is Einstein's fervent denial of entanglement in quantum physics. In other word

It's just a maxim, it doesn't apply in every case. I think the motivating example is Einstein's fervent denial of entanglement in quantum physics. In other words, he's saying the tendency of scientists as they get older is to be more resistant to new ideas. Whether that's true or not in general, is debatable. But it wasn't intended as an ironclad law in the first place, just a tendency