It's an easy thing to connect age with sexual development, but sexual characteristics aren't completely... how to put this... consistent in terms of development with age. Some girls are remarkably developed even at age 13. There's a reason that thirteen was considered marriageable age in the past, you know.
Not that I'm saying it's okay to marry 13-year-olds or anything like that. However, since this is the Internet, I just know that somebody here is going to say "oh so you're saying it's okay to have sex and marry 13-year-olds". Which is not what I'm saying. At all!
It's ludicrous to consider teenagers as completely sexually disconnected anyway. Do they think that teenagers find each other hot because everybody suffers from temporary pedophilia during their teenage years?