There is no hard-and-fast minimum age for characters, but those under the age of 10 may require more justification as to how they'll be able to function within the game, depending on how independent and self-sufficient they are in canon--for example, Takeru/T.K. from Digimon wouldn't require any extra justification to play because he's shown to be very capable of functioning independently in canon.

Characters are considered adults at age 18.