I just had this little interchange with my four year-old:
"Daddy, how come grandma and grandpa are so old?"
"Um, what do you mean?" I asked.
"Well, they were born first, so that makes them old."
"Well, they were born a long time ago, and when they grew up, your mom's mommy and daddy had your mom, and your dad's mommy and daddy had your dad. And since we've grown up and now have kids, that makes grandma and grandpa old." Yeah, I'm often helpful like that.
His brow furrowed in a confused look, then he asked, "Why?"
"Well, that's how it works. We all are getting older all the time. So when you grow up, I'll be old."
"So, when I grow up, will you be my grandpa?"
"No, but when you have kids of your own, I'll be their grandpa."
Then he gave me a huge smile, and said, "Thanks!"
I giggled. "My pleasure, kid."
A moment goes by, then he asked, "How come we have three kids?"
"That's all Heavenly Father has sent to us so far."
Silence, then car noises resumed as he played behind me. Based on this interchange, he probably now thinks that if or when my wife and I have another kid that he'll be its dad!