I don't remember with DD, but she started with the why questions at around 3-4 I think.
I was worried about DS because at 4-5, he still hardly ever asked why, and when he did it was usually me saying something like "We can't go outside right now." and he would say "Why not?" It wasn't questions like "Why does the earth orbit the sun?" or "Why do police cars use sirens?" At age 6-7 we started getting more "Why" and "How" questions but he seems content to read books and ponder things on his own rather than talking/asking about things.