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.
Lilly is portrayed by Emily Osment, the lesser-known - and of late, arguably better-known - sister of HaleyJoelOsment. Some people remember her as Gerti from Spy Kids.
In the Japanese dub: Hannah Montana is Hayate Ayasaki. Incidentally, her voice actress dubs Miley Cyrus in Japanese in almost every other movie or TV series she appears in.
