The "from . . . to" construct is adult.
"personally believe" is also an adult phrase.
"Would have realized" is not only an adult phrase, it is grammar far more complex than that used in the rest of the article. An accepted marker (or "red flag", if you prefer) for this sort of writing is incorrect usage in easy grammar while using the more difficult tenses correctly.
So the internal inconsistencies are in (1) phraseology (2) grammatical constructs (3) reasoning.
There's your internal evidence. We have no external evidence because all we have is the article. So if you demand external evidence, you're out of luck. However, the textual analysis here yields a threefold proof of "two voices" - and my money is on the first voice being the adult author and the second voice being the adult author trying (and failing) to sound childlike.
Another possibility is that the article was originally written by the child, but was so poorly written that it could not be used and was heavily edited by another adult or the editor. But the adult goal - i.e. to paint parents as incapable of raising their own children while promoting sex education - mitigates contra.
I don't think her age is knowable from the evidence. I think it is her message that is unnerving to some. Who she is we really don't know.
I find it hard to believe that any 15 year old lamenting the loss of her virginity would even think to mention the lack of Sex-Ed at her school let alone keep shoving the issue down the readers' throats. IMHO, if this article had truly been written by a 15 year old, she would've talked much more about how the experience affected her emotionally. Most of the emphasis would've been on how she felt dirty or used or disappointed in herself or something to that effect.
Most people who write an "I wish I'd never..." article usually include some sort of advice or warning to others who may be contemplating making the same mistake. If the author is for real and truly regrets her mistake, where is her plea that others not do the same?
Here's the gist of the article: The writer was taught abstinence by her parents. Cultural pressures made abstinence "impossible" for her. She knew absolutely nothing about sex, birth control, or STDs. She had sex, was sent away by her parents to a group home, became addicted to sex because she felt unloved, wishes she'd never had sex, and her biggest lamentation is the lack of Sex-Ed classes in middle school.
What a load of cr@p!
This article sounds like something straight from a Planned Parenthood brochure: Backwards, uptight parents refusing to talk about sex and unrealistically expecting abstinence. Ignorance about sex being a major contributing factor to promiscuity. No mention made of having any moral regrets. Sex-Ed classes being the light at the end of the tunnel. Blah, blah, blah...
That's why I don't believe it was written by a 15 year old. It was written by a shill for Planned Parenthood. Because that's what the article is. It's a piece on the virtues of Sex-Ed, not on the loss of a young girl's virtues.