Someone asked about legality. Not my area of practice(thank God), but the two factors that can be asserted in defense(in the US, anyway) are 1) possessing artistic merit(it doesn't have to be Shakespeare, just something not solely aimed to cultivate prurient interest); and 2) the underage characters are not graphically/visually portrayed in any remotely "realistic" fashion.
Using a high res 3d model with detailed skin textures and making it fairly obvious they were well under the age of consent, and doing so in a game that's basically a "meet and fuck" scenario is a great way to invite unwanted scrutiny and potential legal jeopardy, though it would appear, at present, that these games have thus far not faced much federal or state scrutiny here in the states.
Basically, if you make a shota game, make sure that the story you tell has some artistic merit, and that the shota character is not portrayed too realistically. If it's somewhat realistic, err on the side of making them look at least a bit older than tween age. My .02.