I'm confused by this statement. Doesn't underage mean not yet old enough to consent? I know that, where I live at least, if you're 17 you can't legally have sex with someone who's 19, but you can if they're 17, too. Or at least no one is going to get arrested if they're both 17.
And to address that other thing you said
I have problems with it.
One, you're talking about girls only. I know that's been the focus in this thread, but there are male prostitutes out there and plenty of underage boys. Just wanted to mention that. And you know that some girls are starting puberty at 10 and 11, right? And that puberty isn't just a one day process, that it takes years before you're finished with it?
Two, just being able to have sex physically shouldn't be the only thing you need. There's a level of maturity you really, really should have before having sex. After all, there are big consequences: pregnancy, STIs and STDs. And there are sexual predators out there so a certain level of awareness about the world is something you should have as well. Yeah, I know, there are some really mature kids out there and some really immature adults. If you're 10 and mature enough to have sex you're also probably mature enough to realize you shouldn't just yet. If you think otherwise you're probably not as mature as you think you are.