People say this as if reactors get build once and then stay the same way. These reactors get upgraded all the time. Here's an example: Isar 1 was one of the oldest reactors in Germany. When a German reactor gets new fuel rods it has to be certified again according to current regulations. Isar 1 needed new fuel rods in 2011, so it got upgraded and certified again. After that new fuel rods were introduced into the reactor. A few month later as consequence of the Fukushima disaster it got "shut down"[1], because it was now "not safe". Which is obviously bullshit, cause the rules on reactor safety for German nuclear reactor didn't change due to Fukushima, opinions did and politicians needed to show that they do something.

[1] It didn't really get shut down in 2011. It got removed from the power grid. So, Isar 1 continued to produce power, which was then promptly discharged into the environment as heat. Only after the fuel rods were burned down it really got shut down.