China Anne McClain (age 13), Sierra McCormick (age 14) play 11-year olds as does Jake Short who at 14 is old enough to be a high school freshman on the normal non-A.N.T. schedule, while Allie DeBerry (age 17) is only two years older than her character Paisley, who's 15 years old.
Sierra, who is 14, more than likely wouldn't be in High School yet since most kids who have a birthday after August have to wait until are 5 at enrollment to start school. However, she was on Are You Smarter Than a Fifth Grader? nearly four years ago. Which would mean she either was in fourth grade, or she started school a year early is and indeed in high school now, theoretically speaking of course. Still a huge example of this trope.
Parodied with the cast of the show within a show, High Heels High, who all look as if they are in their 30's.
Most of the high school students you see in the background at Webster High look as if they fall into this trope.
Cameron's actor is 18 but looks much younger, so young, if his voice wasn't as deep as it is, (and it's not that deep) he could have probably been cast as and A.N.T. Chyna is roughly the same height as he is while being 4 years younger, and the actors have about a 5 year difference in real life. That could turn out to be very interesting and funny in later seasons since he is the only main cast member that is pretty much done growing.
He was even mistaken for an A.N.T. on the show on a couple occasions.
Irony as She Is Cast: Allie DeBerry as the Dumb Blonde Paisley is actually said to be very smart. Justified as actresses such as Reese Witherspoon and Amanda Seyfried, as well as many others have proved in the past that it takes a smart actress to pull off the dumb blonde, especially considering roles that the aforementioned actress has done in which they weren't a dumb blonde. In addition to that, she previously sang in a choir. And while she isn't said to be a bad singer, she is the only member of the cast who hasn't sang at all aside from Jake Short (Olive singing the Slovakian national anthem barely counts as singing) on a show with a lot of singing on it.