Just a few days ago I had an interesting conversation with my 4 year old son that went something like this:

"Daddy, what's the biggest number?"

"There is no such thing as the biggest number; numbers keep getting bigger and bigger forever. This is a concept called infinity."

"Which number is the biggest?"

And so on; he's only 4, so I don't expect him to grasp the concept of infinity, but I was impressed that he's already asking about it.

My son is in his first year of Montessori education, and has learned to count into two digit numbers already, and understands zero, but doesn't yet understand negative numbers. Montessori is fairly interesting because they teach math in a different way, by learning powers of 10 and using a bead system similar to an abacus, so Montessori educated kids can actually do simple arithmetic with large numbers (in the thousands) by around age 5.