Here's a really interesting article that suggests life - human, other mammals, amphibians, birds - lasts one billion heartbeats. If you do the math and figure on 70 heartbeats a minute, that's 100,000 heartbeats a day, which only gets you to 30-35 years. Of course, that's pretty much what we'd be living if it weren't for good nutrition, modern medicine, etc.
This reminds me of a great book by one of my favorite children's authors, Andrew Clements. It's a picture book called A Million Dots that visually represents what a million is. When I did my student teaching, I taught a unit on astronomy and I realized that the numbers we were talking about were so outrageously large that they really didn't have any meaning for the fifth graders. I showed them the math on counting if you counted one number every second without stopping, ever (which of course you can't - it takes a long time to say "nine hundred seventy eight thousand, four hundred seventeen"). I had them guess how long it would take for them to get to a billion. It would take almost 31 years! That worked with fifth graders, but for younger students, A Million Dots is a great choice.