I think it's going to be different for everyone. When I was 15, my boyfriend at the time (who I would eventually marry) was staying the night at my house. My dad did find me in his bed one morning (he was supposed to be sleeping in one room and me in another) and completely flipped sh*t. Eventually he moved in because his parents kicked him out of their house. We didn't go to the same school either and he was a senior when I was a freshman. We lived in my parents house until we divorced almost 7 years later.
Now, my niece, who is 17, is in the same situation and her boyfriend, who is 18, and I find it completely weird. He's always here and I'm like, "Don't you have your own home?" Haha!
For me now (24), whenever DB visits (30), he stays in my room and my family makes jokes about what we do in my room. I believe that I got a text on Christmas from my niece that said, "Stop having sex and come open presents." When in fact we hadn't even woken up yet. And when I go to visit him, I sleep in his room. Thankfully, his mum is a heavy sleeper.
So the point is, I really think it's different for each individual person's situation. But even though I did it at such a young age, I don't think that if ever I was in that situation that I'd let anyone under 18 spend the night.