How Do We Know How Long Stars Live?

A camper recently asked me how astronomers know how old stars are. I couldn't think of the answer in the moment, causing me to wave my hand around in distressed circles, but then it magically came to me while I was sleeping that night! I still need to tell the camper the following:

The answer boils down to: We know how big stars are. We know how quickly they burn. Therefore we can figure out how long it takes for them to burn themselves out!

Here's a terrific and more thorough (but not exhaustively thorough) explanation:

http://curious.astro.cornell.edu/about-us/51-our-solar-system/the-sun/birth-death-and-evolution-of-the-sun/167-how-do-you-calculate-the-lifetime-of-the-sun-advanced

And here's a fascinating article that explains that, although we know how long any star's lifetime lasts given its mass, it's actually not possible to determine in general how old a star is if it's not part of a population of stars that were born together (a cluster).

https://www.scientificamerican.com/article/how-do-scientists-determi/

Which led me to ask, how do we know how old our sun is since it's not part of a cluster?!

Although I think there are other lines of evidence that point to our sun's ~5 billion year age, the main one is that we believe our whole solar system formed at the same time (since we believe all solar systems form together in a protoplanetary disk; click the following link to read about how we have recently acquired the first image of a baby exoplanet forming around its baby star!) and we know from radiometric dating that the oldest rocks on earth and meteorites (which were never subjected to the earth's rock cycle and hence have never changed) are about 5 billion years old... Anywhoo, here's a pic of the 10 million year old baby planet! It's the adorable yellow blob below and to the right of the blacked out baby star.

Also, this whole episode reflects how much more interesting it is to learn HOW we know what we know than to just be told things. The fascinating, creative and challenging part is the HOW. For most of human history curious people would have gone crazy to learn that you could actually figure out how long stars live. I can just see people pulling their hair out of their scalps, exclaiming "NO WAY!!! HOW DID YOU FIGURE OUT HOW LONG STARS LIVE?????!!!!!!!"