Jason Earles, who plays Jackson, was a real-life example of Vague Age, his year of birth variably being reported as either 1977, 1985 or 1988, until it was finally confirmed to be 1977, making him almost 29 years old when the show premiered. Averted with the rest of the young characters, who are played by real teenagers.
Of special note: In the episode which teased a Lilly/Jackson pairing, Emily Osment was 16 and Jason Earles was 31, or almost twice her age.
Season 4's recurring guest character Siena, implied to be around Jackson's age, is played by 26-year old Tammin Sursok.
Actually her age was never mentioned. There's no reason she can't be 26.
The show began as a That's So Raven spinoff called Better Days, about a child actor masquerading as a schoolgirl, and it was to star Alyson Stoner.
The lead character was to be named Zoe Stewart, then Chloe Stewart as they felt "Zoe" sounded too much like the lead in Zoey101. They settled on "Miley Stewart" so 13-going-on-14-year-old Miley Cyrus wouldn't have to remember so many names.
At one point the series was under the working title "Alexis Texas". Whether or not the change had to due to a porn actress of the same name who also made her first appearance in 2006 is likely remain a mystery.