Would you believe the meaning of a sentence can be changed drastically by where you place the word only? (Yes, as a matter of fact, I did have a post about this the other day. Preparing this post is what made me realize the topic needed to be addressed in more detail.) The example is saying that people grow old and die when they find their purpose, but that nothing else happens… Change the word order, though, and we can make the same sentence say what it’s supposed to mean: people don’t grow old and die until they find their purpose.
There is no such thing as a millennia (note the spelling: two n‘s, not one), because millennia is plural; the correct singular form is millennium.
People grow old and die only when they have found their purpose in life. You have lived for a millennium when you notice a strand of grey hair.
(Due to fiction projects Mercenary Proofreader has been working on lately, there is stronger-than-normal temptation to snark about individuals who think a millennium is an especially long time… 🙂 )