I don't think 6 is too young at all. There are loads of book you can use. Just deal with it as you would any other question. A few tips here
"Always ask more questions before you offer any possibly over-complex information. Kids are sometimes simpler beings. And very literal. You could find you've just done the whole shenanigans and they only wanted to know where the baby grows. Which, if the child is very young, it only needs a finger pointing to the tummy. Don't rush to offer all the facts, but keep an open conversation going. Till when? Till they stop asking, be it 18 or 80!"