JWD - kids are naturally very curious and inquisitive and are likely to ask all sorts of questions, more or less as soon as they can talk! It is not at all unusual for a five-year-old to ask such questions and I think you are right in being open with from the start.
One of my kids is adopted and we also introduced the idea that she had another mummy when she started asking 'where did I come from'. She is now adult and is glad we told her right from the start.
If you want good communication with your kids as they grow up, you are best to be honest and open from the outset, especially with the more difficult topics.