Yes. "seems normal" is a vague and expansive enough category, particularly applied to the teen population, that certainly some within that range are in fact on the autism spectrum. Often for any condition time must pass before those with more subtle features, or that have shifted into circumstances allowing better recognition of symptoms, will be diagnosed.
There's no such . Thing as " normal, " just neurotypical or average. A teen with any degree of autism will manifest deficits in early adolescence, when social demands become more complex, language becomes rife with figures of speech, slang & innuendo, & academic demands include inference & conceptual thinking.
Not likely. While there is a great deal of diversity in terms of the severity of autism spectrum disorder, a "normal" teen is pretty much likely to be a "normal" teen. Asperger's syndrome is at the far end of the spectrum that is considered "higher functioning;" but there would still be noticeable socialization differences between someone with asperger's and the majority of their peers.