Speculation on TVtropes is that Ricken isn't actually that much younger than the others, he just looks like it, which is a bit of an insecurity for him.

Yeah, I think one of the Lissa conversations says that Donnel's the youngest in the group.

I thought it was obvious. The game often references Ricken's shortness and his desire to become taller, stronger, and treated like any other person his age.

You mean sorta like with the Fire Mage from Radiant Dawm, whose name eludes me....

Tormod.

Wait does anyone know how old Lissa is. I'm guessing around 20.

Probably. Most of the characters appear to be in the 20-30 range.

Though I wouldn't be surprised if at least one was 18 or 19 years old.

I wouldn't have known. I shipped him with Olivia, which mostly had her telling stories and him being scared of the stories she told.

Today I visit a friend and try to sell him on Fire Emblem while I get the chance to play Ni No Kuni.
Tomorrow, I'll see if I can get my mother interested in a non-Wii series Nintendo game for the first time since Wind Waker.
Later this week I'll see if I can visit another friend who I already let play the game (he got past Chapter 2) and has since tried out Radiant Dawn at another friend's house.
Next month I'll be visiting another friend. He actually owns a 3DS, but doesn't have a lot of money to buy games. It's his birthday, so I plan on giving him my copy of Advance Wars: Days of Ruin.
I have one online friend who really wants to try Awakening, but doesn't have a 3DS yet.
I have another online friend who I don't talk to often, but responded positively to my description of the game as a "badass game for badasses".

By the end of the year I expect to have recruited quite a few new Fire Emblem fans.

Given Japanese culture usually have young heroes, I figured at the start of game Chrom and a few others were 16-18 and after the mid game time skip were 18-20. Fredrick and a couple others are probably supposed to be in their mid 20s or early 30s with Ricken, Donnel and Lissa probably starting the game off at 14 or 15. Basillo and Gregor are probably the only characters close or over 40 that aren't magical in nature like the shape shiftters.