Why do parents say stuff to protect young kids but now their kids are older teachers they finally tell them the brutal truth?

My dad said, "You guys are older now so you're old enough to hear the truth. Hey, you're a young adult now. I can say it around you. I can say adult stuff around you."

My oldest bro said, "You better get used it sis, you're going to be hearing adult stuff and you should be. You going to hear and seeing disturbing things. You're not a seven year old little girl anymore. You're a grown ass woman. Start acting like it!"