...with our kids. But how soon is too soon?

After reading an article today about the median age for girls to have their first sexual encounter to be 14, and after speaking to a friend who si today off to buy her 16yo daughter a pregnancy test, I (semi) jokingly said to my kids that I didn't want them to be pregnant, or get anyone pregnant, until 22 as a minimum. DS1 (aged 7) piped up "so that means we can't have sex until we are 22". I have spoken to him about how babies are made before but I was a bit taken aback with how confident and non embarrassed his reply was so I just replied that I didn't mean that, but that he would have to use protection if he was going to have sex. My younger children were in the room at the time.

I know myself that I have not mortified my kids (or given them any details they didn't want) and I hope that they can continue to be open and honest with me about sex as they mature. But am I unusual in speaking to my kids so young about sex?