Playable characters, also referred to as party or playable cast, are the characters that can be controlled by the player. The EarthBound series has various different playable characters with each installment having a new set of playable characters. Most characters are young kids or teenagers, although in Mother 3 adults are playable as well.
The cast of various playable characters, plus Flying Man, Porky, and Dr. Andonuts
Excluding Teddy, the ages of the cast are found in the Mother Encyclopedia:
Ninten: 12
Loid: 11
Ana: 12
Notes
Only 3 characters can be in the party at a time. Normally Teddy replaces Loid once the player reaches Ellay, and then Loid replaces him on Mt. Itoi. Loid is the only party member required to beat the game and Teddy and Ana are optional, using sequence breaks there are different configurations for the party the player can have at the end of the game:
Ninten, Loid, Ana
Ninten, Ana, Teddy
Ninten, Loid
Ninten, Ana
Ninten
Additional character
In addition to this, there is also a temperory party member that is playable in game, who is:
Pippi is a 7 year old girl who temporarily joins Ninten at the beginning of the game. She is interestingly the only temporary party member in the series who can be controlled by the player. Another interesting fact is her stats are identical to Teddy's, this isn't noticed easily at first because Teddy joins at level 18, unlike Pippi who joins at level 1.
Unlike EarthBound Beginnings the ages were left vauge by Shigesato Itoi.
The EarthBound Player's Guide and official UK website claims Ness is 13, however when asked about his age Shigesato Itoi said he never had an official age, and instead compared him to his daughter who was in 4th or 5th garde at the time, implying Ness might actually be younger than what the player's guide and UK website claims.
the UK website says Paula is 11 years old.
Jeff is the same age as Ness so he could be 13 years old or younger.
Poo is left vague, with the player's guide saying he is "a little older than Ness".
Flint is the main character in Chapter 1, where he is searching for his wife Hinawa who was killed by a Mecha Drago. Later his older son Claus goes to the mountains to get revenge on the Drago who killed Hinawa, and Flint goes off to search for him, but Claus was nowhere to be found.
Salsa is a monkey who is playable in Chapter 3, where he was captured by the Pigmask Army and is fourced to work for Fassad in order to save his girlfriend.