Still no joy with this. SyzygyΣE suggested turning "Radius from Composition" off then back on again before aging, but that didn't seem to work. I'd really like to know if this is a bug or if I'm doing something wrong, is the time-step simply to large? Any help would be much appreciated.
Notice that age returns to 0 after switching RfC off and on, is that supposed to happen? The resulting supernova remnant doesn't look at all like I'd expect and is giving off what appear to be flares, that doesn't seem right either.
Thanks for presenting a video. It makes it a lot easier for me to test things. I'll explain what I can gather from your simulation.
First of all, it should be noted that the sun doesn't actually have enough mass to explode in a supernova. Instead, at the end of its life, it will throw off its outer layers in what is known as a planetary nebula, leaving its core—the remnant—behind. As of now, planetary nebulae are not simulated in the game. That's why you aren't seeing any sign of a nebula being produced, and instead seeing flares. The fact that the name of that object reads "Sun Nova Remnant" is actually somewhat misleading, because it isn't any kind of nova at all. You are not to blame for mistakenly referring to it as a supernova.
And yes, when the remnant is left behind, the age is supposed to reset. This is because the game considers the remnant a new class of objects, and not related to the star it was born out of. There also seems to be a separate issue: the fact that the remnant of a sun-like star has the same radius as the actual star. The leftover core in a planetary nebula event is much smaller than the star it once was, but the simulation doesn't change its radius until you toggle the "Radius From Composition" switch.
I do not seem to be encountering the issue you described. I have attached a video of the stellar evolution process at work on the sun, including resetting the radius of the remnant object. Try to follow my steps and see if you can get it working.