I have never met anyone that I was sure was an adult in every way. We have a pretty clear definition of kid nowadays that says adulthood begins at 18. I am glad I had sex before 18. I certainly was not an adult in everyway, but I don’t know how I would have made it through to 18 without that relationship.
I did know one woman who had a child at 15, and then a few more. She was married to a 25 year old. Though this is not how I would choose my daughters life to be, it did seem to work out pretty well for my great grandmother. She loved her husband and even when I knew her far after her girlhood, that she still thought about sex as something fun, amusing, and relevant to her was evident at times in her words and actions. I know of no reason to suspect she was that much more grown up when she was married than I would have been at that age. She did speak of putting her dolls away when she was courting.
Broad generalizations, the statement in the OP included are not worth the breath they take to say. This one is insulting and shortsighted at best.