Little Boy and Big Thinker

My son is just 3 years old. But he has many questions now. Some questions are very interesting and are hard to answer.
One day, he asked: Dad, when I grow up, how about you?
I said: I will be old.
Then, he asked: How about you when I am old?
I thought for 1 second, and answered hardly: I will die.
He didn't say anything for this and kept playing with his toys. After 1 minute, he said suddenly: It will be great if we live forever and never die.
I didn't cry at that moment because I am an adult.