This topic is still alive?? K, here goes. I might as well put off finishing up my work for another half hour.
I seriously doubt a bunch of executives sat around focus-grouping the ages of the characters. (Other than the precursor to Idolmaster, which apparently did yield some useful information.) What's more important are the ages of the characters relative to each other, so that senior/junior relationships can be charted out or inverted.
As for why they skew as young as they do, I can think of a couple reasons.
- As the producer, you are put in charge of a struggling studio. It makes sense for the idols to be new and inexperienced, so you can guide their development without having to unteach them. If you were handed a group at the tail end of their sputtering careers, it would be rather depressing.
- As the franchise progresses, it is easier to age the idols up than down (and having to support or argue away prequels), so starting them off a couple years younger leaves them a buffer. (Look at what happened to Rorona of the modern Atelier series. By the third game, Meruru, the developers couldn't cope with the idea that she was hitting her 30's, so they turned her into a 5-year old girl. This, despite supporting cast members hitting their 40's).
- I could make some vague generalizations about Japan's obsession with youth culture, though I think some of them can be turned back upon us.
- Idol careers are apparently very short, from my limited understanding. If they don't catch fire, they're done, and if they hang around long enough, they eventually overstay their welcome. I believe that was the premise of the original Idolmaster; after one year, your idol's career is over, that's it. Though short careers mean an older cast is viable, would you want another reminder that forced retirement is just around the corner?
- Finally, I think designers occasionally make small errors in judgement when it comes to character traits, be it age, weight, or measurements. If you have three different people, one writing up character metrics, one doing concept art, and one doing video game sprites or models, characters can wind up looking heavier, shorter, older, etc. than their declared stats.