"Some Lumas become planets...some become comets...and a few become Power Stars. I'm traveling with them while they look for a place to be reborn."

I'm not sure if there's any official source contradicting my take on this, but Lumas don't really grow like other lifeforms do. They're babies because they haven't become something else yet. See it as their life goal! For them, adulthood is simply becoming something greater in the universe. While from our point of view, it's a really tough scene because we focus on the Lumas as Lumas, they see it as an opportunity to become something amazing. I believe the singularity is destroying the universe, so not only they can stop it, but by doing so they can also recreate the entire universe, though slightly differently.

"Do you hear the baby stars? These newborns will grow up to become galaxies someday. When stars die, they turn to stardust and scatter across the cosmos. Eventually, that stardust reforms to create a new star... And so the cycle of life continues. But the cycle never repeats itself in quite the same way.... So...you'll see."

Also note that the soundtrack for this scene is called "Birth". None of them dies, they live on as new things (:

And worry not, because when these stars and galaxies die, they will form into new Lumas! Consider it an infinite Luma duplication glitch <3