Why is so many claiming that life is too short when it really isn't, when it takes forever for a child to become an adult.?

When you're 80 you'll understand what they mean.

18 to 21 years is NOT forever. Even to you kids.

Forever is a subjective term. Human lifespan is short when you compare it to that of a diamond.

When you look back when you're old it seems short.

You will understand when you are older, but I'll explain it to you. Every year after you're 30 will go faster and faster; and after you've lived through the most of your life you'll look back and it seems like only yesterday when you were a child but now you see the day ahead of you when you will be dead like a deer looking into the headlights. Not that I'm complaining; the pleasures of life are too much outweighed by the burdens to me.

The entire life of every individual is less than a blink of an eye in the grand scheme of the universe.