As for why Mario's older, it's exactly what LeftyGreenMario and Baby Luigi said: There's going to be one twin who's slightly older, even if it is only by a couple of minutes. A pair of twins I know were born three minutes apart, so one of them is three minutes older than the other. I'd guess some apparent differences between the age may have been caused by Nintendo flip-flopping between whether they were twins or just brothers earlier on, though I think Mario and Luigi look the same age, but I've come across a fair few people who think that Luigi is the older of the two. Their age can look slightly different if you were to compare artwork from two different games, but they always look the same age if you compare the two from the same game. I've always though that Luigi acted slightly younger though, but I think that's more part of his personality.