Although they would deny it, my parents were pretty overprotective, which meant I didn't date much in high school. As a result, I jumped into a relationship with one of the first guys to come along when I was in college. I knew that my parents hated him. They never said a word, but I knew. I'm glad they didn't push the issue because, as a fairly immature 19-year-old, I probably would have grown closer to him out of spite.
However, I'm certain they would have said something if we had gotten engaged or moved in together, and rightfully so. There is a difference, IMO, between interfering in an official but relatively casual relationship and something more permanent if there truly are red flags. I'm not saying that's true for everyone, but I would expect those who care most about me to speak up if they think I'm about the make a huge mistake. Maybe they're overstepping, or maybe they're seeing something I'm not.
With that said, I not would allow my hypothetical 17-year-old daughter to date someone who's 21. Even as an 18-year-old freshman in college, I would have looked askance at a guy of that age who wanted to date a girl in high school. The difference in circumstances and life experience at that point are pretty dramatic. YMMV.