At 12 I was 5'9 with wide hips and F cup breasts. I also had a fairly deep voice, no hint of a baby face, and because of my height and curves, an adult woman's wardrobe.
People regularly confused me for someone in their 20's. When I went places with my 9yr old sister, people assumed I was her mum. At school new teachers and students would always mistake me for a teacher despite the uniform. I could go to R rated movies, go to clubs and buy alcohol without issue. At 13 we moved to the US for a while and I had no problem getting into some very strict 21+ establishments.
If I'd said I was 16 the part people would have questioned was that I wasn't older. There's no way anyone could have known my real age without asking for ID, and people don't tend to do that when hooking up with someone unless the other person actually looks underage, which apparently this girl didn't.