I was Googling about the oldest galaxies in the Universe. Everywhere is written that their age is known by the light. Referring to this line "Since light travels at a set speed, if you look at a star ten light years away, you're really seeing what it looked like ten years ago" on this site.
So my question is, as we know light is continuous, how do scientists know that a particular ray has started its journey some 1000 light years back? How is the age of stars determined?