TPIR: Required to even be in the audience is 18. Also 18 is the requirement for contestant selection. Also...you can't be on any version of TPIR in the last 10 yrs (there's a very unique story that goes with that). The closest to an all-teen edition was a college-student only Million-Dollar Spectacular, which, in TPIR's mind, laid the big egg...only one game was won, and I believe there was a double overbid in the showcase. In other words, a disaster.
Millionaire: 18 yrs old. And I prefer Meredeth's version to Regis's.
Wheel: 18 yrs old as well.